Imágenes de página
PDF
ePub

and we knew that he would not quit it till he had either discovered its insignificancy, or had made something of it. He learnt the German language in order to peruse Leopold's Theatrum machinarum. So did I, to know what he was about. Similar reasons made us both learn the Italian language. * * * When to his superiority of knowledge is added the naïve simplicity and candour of Mr Watt's character, it is no wonder that the attachment of his acquaintances was strong. I have seen something of the world, and am obliged to say I never saw such another instance of general and cordial attachment to a person whom all acknowledged to be their superior. But that superiority was concealed under the most amiable candour, and a liberal allowance of merit to every man. Mr Watt was the first to ascribe to the ingenuity of a friend, things which were nothing but his own surmises followed out and embodied by another. I am the more entitled to say this, as I have often experienced it in my own case." * It is for you, gentlemen, to determine whether it was not as honourable to have expressed this concluding sentiment as to have inspired it.

The diversified and profound studies in which the circumstances of his singular position unceasingly engaged the young artisan of Glasgow, were never allowed to interfere with the labours of the workshop. These he executed during the course of the day, whilst the night was devoted to theoretical researches. Confiding in the resources of his fertile imagination, Watt appeared to luxuriate in the most difficult undertakings, and in those which might be thought most foreign to his tastes. Will it be believed that he undertook the building of

*MS. of the late Professor Robison.

"

an organ, though totally insensible to the charms of music, so much so that he could not distinguish one note from another? Nevertheless the work was accomplished. It is scarcely necessary to say, that the new instrument exhibited important improvements in the mechanical details,-in the regulators, and in the manner of measuring the force of the wind; but one is surprised to learn, that its powers of harmony were not less remarkable, and that they delighted professional musicians. Watt, in fact, resolved an important part of a very difficult problem; he made out the scale of temperament by the method of pulsations (des battements), at that time little understood, and the knowledge of which he could not have obtained except in the profound but very obscure work of Dr Robert Smith of Cambridge.

History of the Steam-Engine.

We are now arrived at the most brilliant period of the life of Watt; and I fear also at the most difficult part of my task. The immense importance of the inventions of which I am about to treat, cannot for a moment be doubted; but possibly I may not succeed in making them clearly understood, without going into minute numerical comparisons. That these comparisons, if they do become indispensable, may be readily followed, I shall here state, as briefly as possible, the abstract physical principles upon which they must be based.

As the result of simple change of temperature, water may exist in three perfectly distinct states,-in the solid, the liquid, and the gaseous state. Below 32° Fahr. water becomes ice, at 212° it is rapidly transformed into vapour, and in all the intermediate degrees it is liquid.

The careful observation of the points of passage from one of these conditions into another, leads to discoveries of the highest importance, which form the key to the economical appreciations of steam-engines.

Water is not necessarily warmer than is every kind of ice; water may be maintained at the temperature of 32° without freezing; ice may continue at 32° without melting; but it is very difficult to believe that this water and this ice, both of them at one and the same degree of temperature, differ only in their physical properties, and that there is not some element, apart from water properly so called, which distinguishes the solid water from the liquid water. A very simple experiment will elucidate this mystery. Mix two pounds* of water at the freezing point with two pounds at 167° Fahr., the four pounds of the mixture will be found to be at 99°, that is to say, at the mean temperature of the commixed liquids. The hot water is thus found to have preserved 67° of its previous temperature, and to have yielded 67 other degrees to the cold water. All this is what would readily be expected, and could easily be foreseen. And, now, let us repeat the experiment with a single modification. Instead of the two pounds of water at the freezing point, let us take two pounds of ice at precisely the same temperature. From the mixture of this two pounds of ice with the two pounds of water at the temperature of 167°, there will result four pounds of liquid water, since the ice, plunged into the hot water, must needs be dissolved, and will yet retain its former weight; but you

* In this illustration, the fractions arising from the differences of the thermometric scales are omitted, and hence the figures are only approximatively correct.-EDIT.

must not conclude that, from this second mixture, there will result as from the former a temperature of 99°. Very far from it; in this latter experiment, the water will not be above the freezing point, and there will not remain a single trace of the 135° of the heat of the two pounds of water; these 135' will have dissolved all the particles of the ice, and have combined with them, but without having heated them in the slightest degree.

I have no hesitation in adducing this experiment of Dr Black's as one of the most remarkable in modern physics. Observe its consequences. Ice, at its habitual temperature 32°, and water at the same temperature, differ in their essential composition. The liquid, in addition to what is contained in the solid, includes 135° of an imponderable body which is called caloric. These 135° are so thoroughly concealed in the compound, I was about to say the watery alloy, that the most delicate thermometer cannot detect its existence. Hence then, caloric, which is not discoverable by our senses, and which cannot be detected by the most delicate instruments,in short, latent heat, for that is the name which has been bestowed upon it, forms one of the constituent principles of bodies.

The comparison of boiling water, that is to say of water at 212°, with the steam which issues from it, and whose temperature is also 212°, leads to analogous results, but upon a much grander scale. At the moment that steam, at the temperature of 212°, is produced, the water, at the same temperature of 212°, impregnates itself,-under the form of latent caloric,-under a form quite insensible to the thermometer,— with an enormous quantity of heat. Again, when the steam reassumes the liquid state, this caloric of composition is disen

gaged, and goes to heat every thing in its way which is susceptible of absorbing it. If, for example, we were to cause two pounds of steam at 212° to pass through ten pounds of water at the freezing point, the steam would be wholly liquefied, and the twelve pounds which would result from the mixture would be found at the temperature of 212°. Into the intimate composition of two pounds of steam, there enters therefore a quantity of latent caloric sufficient to raise two pounds of water, whose evaporation is prevented, from the freezing point to the height of 995° Fahr. This result will without doubt appear enormous, but it is quite certain. Steam exists only upon this condition. Wherever two pounds of water at the freezing point are evaporated, whether naturally or artificially, in undergoing the transformation, they must seize upon, and in fact do seize upon, 995° of caloric derived from surrounding objects. This number of degrees (for it cannot be too often repeated), the steam entirely restores to the surfaces of whatever nature, upon which the condensation is ulteriorly effected. And here, we may remark in passing, is the whole secret connected with the art of heating by steam. That individual would have a very erroneous idea of this ingenious contrivance, who supposed that the steam conveyed to a distance in the tubes in which it circulates, nothing more than sensible or thermometric heat. The chief effects are, beyond all doubt, owing to the caloric of composition-the hidden or latent heat-which is disengaged at the moment when the contact of the cold surfaces converts the vapour from the gaseous into the liquid state. We must therefore rank caloric among the constituent principles of steam. Caloric is obtained only by the combustion of wood, coal, &c. Steam therefore has a commercial value superior to that of the

« AnteriorContinuar »