obtained from water by heat, by the air-pump, &c. to corroborate the absorption. The best and readiest method of all is to expel the atmospheric air by hydrogen, in which case the air expelled and the hydrogen absorbed are immediately determined by well known methods; but to those who do not understand the theory of this process it can be of little use. The quantity of oxygen in water is accurately ascertained by agitating with nitrous gas, as I stated in the paper 12 years ago, allowing 3-4 nitrous for 1 oxygen, or perhaps more correctly 3.6 for 1. The point of saturation is when neither of the gases is found in the residue. Another very easy method of determining the oxygen in water has recently occurred to me, and in about a dozen trials I have found it never deviate, though made upon very different quantities of water, namely from five ounces to 200; it has one advantage too, that it requires no great skill either to execute or comprehend it. I will relate one of the experiments.

18 ounces of clear rain-water and three of lime-water were briskly agitated together in a tall cylindric jar, so as to acquire a full charge of atmospheric air; after standing a few minutes 13 grain measures of a solution of green sulphate of iron (1∙157 = 32 salt = 1 oxide) were put into the water and gently agitated with a rod for five minutes. In half an hour a pure yellow oxide had subsided. The water was drawn off by a syphon, and seven grains more of the sulphate were put in and agitated as before; in a quarter of an hour a perfectly green oxide had subsided, which preserved its colour for many days under the water. Now if Saussure's estimate of oxygen in water be admitted, the last should have been a perfect yellow oxide. Allowing of the weight of the green oxide for the additional oxygen, we find about 80 grain measures of oxygen gas in 21 oz. of water, or 10080 grains. the water had been charged with pure oxygen the quantity would have been 400 grains, or nearly four per cent., instead of 6.5, and five times the quantity of sulphate would have been required, as I have repeatedly found. No oxygen is found in water after the green oxide begins to be permanent.

If

Though the quantity of oxygen gas absorbed by water appears to me decisively to be 37 per cent. as I before determined, or perhaps 4, when recent agitation in the purest gas has been used, yet I do not find the same degree of accuracy in my former results in regard to azote; I have repeated some of my experiments and find that water takes 24 per cent. of azote as nearly as possible, and not 1.56 as I formerly stated, nor yet 41 as Saussure states. Both Henry and Saussure are wrong I believe in placing azote below hydrogen; this last is the least absorbable; water takes very nearly two per cent. of hydrogen. Henry says, 1.6, and Saussure, 4'6'; but it must be understood that both of these chemists profess to give the observed absorption by water purified by boiling only, and not absolutely pure; whereas the numbers I have given above are understood of absolutely pure water. If we allow that part of

the air still remains in water after long boiling (which is no uncommon circumstance); then Henry's numbers would be 19 for hydrogen, and Saussure's 5.5, making an enormous disproportion in their errors.

When water is violently agitated with any kind of gas, it is some time before the surplus gas mechanically diffused through the water in small bubbles makes its escape: after all there is probably a surcharge; the quantity of the surcharge I have endeavoured to investigate; it is an object of some consequence in a theoretic point of view. The following may be of some use to such as choose to take up this subject. When a gallon of water (or about of a cubic 14 foot) is well boiled, and then poured into a cylindric jar, so as to be about eight inches deep, and suffered to remain exposed to the atmosphere without agitation, it recov one half of its air in two days. In 10 days no further absorption is remarked; but by violent agitation it will take or of a full charge. It should be known that water requires about one minute's agitation to acquire a full charge of any gas, when previously as free as possible from air, supposing the water to be about 20 times the volume of air; but if the water be 30 or 40 times the air, it may require two or three minutes' agitation. I seldom allow less than five minutes.

It is not very obvious to me why Saussure has passed over an important feature of the theory of absorption without any notice I can find. I mean the effect of heat. He has some allusions to it in the paragraph announcing his modes of freeing liquids from air; but they are evidently accidental, and he enters upon no explanation. He suggests two methods of expelling air (or rather suffering it to escape) from liquids; the one is, by removing the incumbent air from their surfaces and substituting steam of equal pressure (that is, boiling the liquids); the other is, by removing the incumbent air and substituting steam of very weak pressure, (that is, by the air-pump.) He observes that in the last case "the pressure of the vapour prevents the escape of the air." It would be very reasonable to ask why the pressure in the former case does not much more prevent the escape of the air. The only answer, I conjecture, that Saussure and you could give, would be that the heat of boiling water expels the air with so much force, that a counteracting pressure equal to that of the atmosphere is insignificant. The truth is that neither the heat nor the vapour has directly any influence upon the expulsion or retention of the air. If a tube half filled with water and half with air be hermetically sealed, it may be put into freezing water, or boiling water, which ever we please, for half an hour, and no air will be observed to pass either into or out of the water, though the pressure of the incumbent steam varies from 1 to 150, and the temperature from 32° to 212°. The same tube open at the end immersed in boiling water would in half a minute be quite opake with ascending air bubbles.

M. Saussure and you have represented it as my theory, that "all liquids" absorb the same quantity of air as water does. (p. 339,

and vol. vii. p. 23.) My words are "most liquids

except."

Those expressions, are, I believe, not generally deemed synonymous. My experiments were much more numerous on water than on other liquids, and what I had more particularly in view was to show that any slight modification of water by acids, salts, &c., such as might naturally occur, was not sensibly distinguishable from pure water in regard to absorption. On concentrated liquid acids and saline solutions I had not made many experiments. A strong solution of common salt I found to absorb only one third of the volume of any gas that water did, and this was the reason of my saying "most" instead of "all liquids. except;" but guarded as the expression was, I am ready to allow that it was not sufficiently so. Saussure's copious experiments on other liquids than water far surpass mine in number and variety, and will be found a valuable acquisition, if the accuracy of the less absorbable gases be equal to that of the more absorbable ones.

If the influence of chemical affinity did not exist, the gases would be absorbed by all liquids in the same order, according to Saussure; and finding them not so, he concludes, that the absorptions are occasioned by affinities. A better distinction in my opinion would be, that if a volume of any gas or mixture of gases is absorbed by water in proportion to the pressure of the incumbent gas, and the same volume is capable of being expelled again unchanged by the usual means of boiling, the air-pump, or agitation with any other gas, then the absorption is mechanical; but if a change in the quantity or quality of the gases expelled be observed, it must be ascribed to affinity: thus when nitrous gas or sulphureted hydrogen are pressed into pure water freed from all air, we can rarely if ever recover the same quantity again, the respective gases being in a short time partially decomposed. If it be said that solutions of ammonia, muriatic acid, &c. in water, must upon these grounds be considered as mechanical combinations; I grant they are combinations of a mixed nature, partly mechanical and partly chemical. The immense condensation of volume of those gases by water cannot be accounted for on mechanical principles alone; the water must have an affinity for the bases of these gases, or for their caloric, or both, and besides the quantity is not as the pressure. But when no condensation of gas takes place, and the quantity is accurately as the pressure, to call this a case of affinity seems to me just as reasonable as to ascribe the air in a sandhill to the chemical affinity of sand for air, and to argue that that affinity varies according to the state of the barometer.

It may not be amiss to sum up these remarks under a few heads, exhibiting the leading principles of the theory of absorption which I adopt, in order that they may be more clearly understood. The gases are of course chiefly those of which water does not take more than its bulk.

1. The quantity of any pure gas which water absorbs is in proportion to the pressure or density of the gas.

This was Dr. Henry's discovery; but I adopt it as an essential principle of the theory. Saussure also confirms it.

2. The quantities of any mixture of gases which water absorbs are also in proportion to the pressure or densities of the several incumbent gases after the absorption has ceased, (but not in proportion to their pressures before the absorption, unless these two ratios happen to be the same); and are the same as if the gases were alone, allowing for the diminished density. Thus, water charged with atmospheric air of unlimited volume contains of a full charge of oxygen, and 7% of a full charge of azote; but if water be charged with a limited volume of air, as, then it will contain less oxygen and more azote than specified above.

79

This was a discovery of mine; it is confirmed by the experiments of Henry, and by the four experiments of Saussure above explained, the only ones of his that apply to it.

3. Heat and cold, or change of temperature, has no influence on the quantities of gas absorbed by water.

This was an observation of mine; the idea was at first suggested by a consideration of other facts, and afterwards confirmed by experiment. As heat increases the force of the incumbent air in proportion as it increases that of the air in the water, the equilibrium is not disturbed. The reason why heat seems to expel air from liquids is that it generates steam, which removes the atmospheric air from the surface. The air-pump, or hydrogen gas, will remove the pressure of the azote and oxygen of the atmosphere, and are equally efficacious in expelling the air from water without heat. This feature of the theory, as has been observed, has not been noticed by Saussure.

4. The quantities of the several gases absorbed by water are as 1, &c.; the volume of water being unity.

1

This observation occurred to me during the investigation, and I attempted to show that these proportions necessarsly resulted from the preceding phenomena. Though many of Saussure's results differ widely from the above proportions, in consequence of their being erroneous as shewn above, yet it must be allowed that some exceptions occur in regard to this law, particularly in the class which should be absorbable: whether the approximations are accidental, or whether they are founded on the principle of equilibrium I have suggested, may be a fair subject for future discussion when the facts are ascertained beyond doubt.

The assertion that I made, that "most liquids freed from viscidity, such as acids, alcohol, liquid sulphurets, and saline solutions in water, absorb the same quantity of gases as pure water, except they have an affinity for the gas, such as the sulphurets for oxygen, &c." appears to me to be too general and comprehensive. Saussure has clearly shown that oils, acids, alcohol, and saline solutions differ very materially from waters in the quantity of gases absorbed; but for any thing that appears these liquids all agree with water in the other three primary laws.

I must now leave it to be determined whether "it would appear from these experiments of De Saussure, that Mr. Dalton's theory [of the absorption of gases by liquids] is erroneous in every particular." I remain respectfully yours,

JOHN DALTON.

ARTICLE VI.

Defence of the Objections to Prevost's Theory of Radiant Heat. By John Murray, M. D. F.R.S. E., Lecturer on Chemistry in Edinburgh.

SIR,

(To Dr. Thomson.)

Edinburgh, Jan. 20, 1816, In the general view you have given in your last Number of the late improvements in science, you mention that Mr. Davenport had written a very complete refutation of some objections that had been started against Mr. Prevost's theory of Radiant Heat. This refers, I believe, to the answer given by that gentleman to some objections to the application of that theory to radiant cold. One of these objections I had advanced, and I now take the liberty of making a few observations on Mr. Davenport's reply to it, which I was prevented by circumstances from doing at the time it was published.

When a tin cannister containing a freezing mixture is placed opposite to a reflecting metallic mirror, in the focus of which a thermometer is placed, if one of the surfaces of the cannister be covered with a coating of lamp-black, it is known that the depression of temperature which is indicated by the thermometer, is much greater than when the clear metallic surface is opposed. This appears to me inconsistent with Prevost's explanation of radiant cold. That explanation assumes that the effect depends merely on the interchange of rays of heat between the thermometer and the cold surface, regulated by the mirror, the thermometer being at a higher temperature, and therefore giving off more radiant heat than it receives in return from the cold body; so that its temperature falls. Now it seems obvious that of different surfaces giving off different portions of caloric by radiation at the same temperature, the one which gives heat will allow of the greatest depression of temperature in the thermometer, for it is the one which will make the least return. A metallic surface is that which radiates least, it therefore should cause the greatest degree of cold when opposed to the thermometer; but it causes the least, and the blackened surface which discharges the largest quantity of caloric by radiation is the one which, in this experiment, causes the greatest dépression of temperature in the ther

mometer.

Mr. Davenport's reply (which has been considered as satisfactory