water before any smoke can have mixed with the air. In order to draw air out of a veffel which has its mouth immersed in water, and thereby to raise the water to whatever height may be neceffary, it is very convenient to make use of a glass fyphon, fig. 13, putting one of the legs up into the veffel, and drawing the air out at the other end by the mouth. If the air be of a noxious quality, it may be neceffary to have a syringe fastened to the syphon, the manner of which needs no explanation, I have not thought it safe to depend upon a valve at the top of the veffel, which Dr. Hales fometimes made use of. If, however, a very fmall hole be made at the top of a glass veffel, it may be filled to any height by holding it under water, while the air is iffuing out at the hole, which may then be closed with wax or cement. If the generated air will neither be absorbed by water, nor diminish common air, it may be convenient to put part of the materials into a cup, fupported by a ftand, and the other part into a small glafs veffel, placed on the edge of it, as at f, fig. 1. Then having, by means of a fyphon, drawn the air to a convenient height, the in a variety of the fmall glafs veffel may be eafily pushed into contents of the The diftance be tween the boundary of air and water, before and after the operation, will fhew the quantity of the generated air. The effect of proceffes that diminish air may also be tried by the fame apparatus. When I want to admit a particular kind of air to any thing that will not bear wetting, and yet cannot be conveniently put into a phial, and especially if it be in the form of a powder, and must be placed upon a stand (as in those experiments in which the focus of a burning mirror is to be thrown upon it) I first exhaust a receiver, in which it is previously placed; and having a glafs tube, bended for the purpose, as in fig. 14, I fcrew it to the ftem of a transfer of the air pump on which the receiver had been exhaufted, and introducing it through the water into a jar of that kind of air with which I would fill the receiver, I only turn the cock, and I gain my purpose. my purpose. In this method, however, unless the pump be very good, and feveral contrivances, too minute to be particularly defcribed, be made ufe of, a good deal of common air will get into the receiver. When I want to measure the goodness of any kind of air, I put two measures of it into a jar ftanding in water; and when I have marked upon the glass the exact place of the boundary of air and water, I put to it one measure of nitrous air; and after waiting a proper time, note the quantity of its diminution. If I be comparing two kinds of air that are nearly alike, after mixing them in a large jar, I transfer the mixture into a long glafs tube, by which I can lengthen my scale to what degree I please. If the quantity of the air, the goodness of which I want to afcertain, be exceedingly fmall, fo as to be contained in a part of a glass tube, out of which water will not run fpontaneously, as a fig. 15; I first measure with a pair of compaffes the length of the column of air in the tube, the remaining part being filled with water, and lay it down upon a scale; and then, thrufting a wire of a proper thickness, b, into the tube, means of a thin plate of iron, angle c, to draw it out again, of this little apparatus has been introduced, through the water into a jar of nitrous air; and the wire being drawn out, the air from the jar muft fupply its place. I then measure the length of this column of nitrous air which I contrive, by bent to a sharp when the whole I have got into the tube, and lay it alfo down upon the scale, fo as to know the exact length. of both the columns. After this, holding the tube under water, with a fmall wire I force the two separate columns of air into contact; and when they have been a fufficient time together, I measure the length of the whole, and compare it with the length of both the columns taken before. A little experience will teach the operator how far to thruft the wire into the tube, in order to admit as much air as he wants and no more. In order to take the electric fpark in a quantity of any kind of air, which must be very fmall, to produce a fenfible effect upon it, in a fhort time, by means of a common machine, I put a piece of wire into the end of a small tube, and faften it with hot cement, as in fig. 16; and having got the air I want into the tube by means of the apparatus fig. 15, I place it inverted in a bafon containing either quickfilver, or any other fluid substance by which I chufe to have the air confined. I then, by the help of the air pump, drive out as much of the air as I think convenient, admitting the quickfilver, &c. to it, as at a, and putting a brass ball on the end of the wire, I take the fparks or fhocks upon it, and, thereby tranfmit them through the air to the liquor in the tube. To take the electric fparks in any kind of fluid, as oil, &c. I ufe the fame apparatus defcribed above, and having poured into the tube as much of the fluid as I conjecture I can make the electric spark pass through, I fill the rest with quickfilver; and placing it inverted in a bafon of quickfilver, I take the fparks as before. If air be generated very faft by this process, I ufe a tube that is narrow at the top, and grows wider below, as fig. 17, that the quickfilver may not recede too foon beyond the striking distance. Sometimes I have used a different apparatus for this purpose, reprefented fig. 18. Taking a pretty wide glass tube, hermetically fealed at the upper end, and open below, at about an inch, or at what diftance I think convenient from the top, I get two holes made in it, oppofite to each other. Through these I put two wires, and fastening them with warm cement, I fix them at what diftance I pleafe from each other. Between these wires I take the sparks, and the bubbles of air rife, as they are formed, to the top of the tube. PART |
