VOL. LXXV.] PHILOSOPHICAL TRANSACTIONS, 1*J 



bility with which this experiment was tried. In the 2d place, the air cooled by 

 the ice above the scale becoming heavier than the surrounding atmosphere, would 

 press on the scale downward with the whole force of the difference. If a little 

 more than half a pint of air was cooled over the scale to the heat of the ice and 

 glass containing it, that is, 20° below the freezing point, the difference, accord- 

 ing to General Roy's table, would have been the 8th part of a grain, which was 

 the weight acquired; but the air within half an inch of the glass vessel being only 

 1° below the freezing point, I cannot conceive, that even an 8th part of a pint of 

 air could be cooled over the scale to 20° below the freezing point ; nor that the 

 whole difference of the weight of the air over the scale could ever amount to the 

 32dofagrain. I have however contrived an apparatus which is executing, in 

 which this cause of fallacy will be totally removed. I shall therefore rest at pre- 

 sent the state of this part of the subject ; and leave it only proved, that water 

 gains weight on being frozen. 



XXII. Sketches and Descriptions of Three Simple Instruments for Drawing 

 Architecture ctnd Machinery in Perspective. By Mr. Jas. Peacock, p. 366. 

 These machines, and descriptions of their use, appear to be too complex and 



operose to be employed in such practical cases of drawing. 



XXIII, Experiments on Air. By H. Cavendish^ Esq. F. R. S. and A, S^ 



p. 372. 



In a paper, printed in the last volume of the Philosophical Transactions, in 

 which I gave my reasons for thinking that the diminution produced in atmospheric 

 air by phlogistication, is not owing to the generation of fixed air, I said it seemed 

 most likely, that the phlogistication of air by the electric spark was owing to the 

 burning of some inflammable matter in the apparatus ; apd that the fixed air 

 supposed to be produced in that process, was only separated from that inflam- 

 mable matter by the burning. At that time, having made no experiments on 

 the subject myself, I was obliged to form my opinion from those already pub- 

 lished ; but I now find, that though I was right in supposing the phlogistication 

 of the air does not proceed from phlogiston communicated to it by the electric 

 spark, and that no part of the air is converted into fixed air ; yet that the real 

 cause of the diminution is very different from what I suspected, and depends on 

 the conversion of phlogisticated air into nitrous acid. 



The apparatus used in making the experiments was as follows : The air through 

 which the spark was intended to be passed, was confined in a glass tube m, bent 

 to an angle, as in fig. 4, pi. 1, which, after being filled with quicksilver, was in- 

 verted into two glasses of the same fluid, as in the figure. The air to be tried 

 was then introduced by means of a small tube, such as is used for thermo- 



