35(3 PHILOSOPHICAL TRANSACTIONS. [ANNO 1783. 



accurate; but if it were not it would not signify, as I determine the exact quan- 

 tity of air used by weight. 



There are 1 different methods of proceeding which I have used ; the first is 

 to add the respirable air slowly to the nitrous; and the other, to add the nitrous 

 air in the same manner to the respirable. The first is what I have commonly 

 used, and which I shall first describe. In this method a proper quantity of 

 nitrous air is put into one of the bottles m, by means of one of the measures 

 above described, and a proper quantity of respirable is let into the vessel a, by 

 first filling it with this air, and then setting it on the knob c, as was done by 

 the measure. The vessel a is then fixed in the socket, and the bottle m placed 

 with its mouth over the cock. Then on opening the cock, the air in the vessel 

 a runs slowly in small bubbles into the bottle m, which is kept shaking all the 

 time by moving it backwards and forwards horizontally while the mouth still 

 remains over the cock. 



Notwithstanding the precautions used by the Abbe Fontana in measuring the 

 quantity of air used, I have sometimes found that method liable to very con- 

 siderable errors, owing to more water sticking to the sides of the measure and 

 tube ax one time than at another: for this reason I determine the quantities of 

 air used and the diminution, by weighing the vessels containing it under water in 

 this manner. From one end of a balance, placed so as to hang over the 

 tub of water, is suspended a forked wire, to each end of which fork is fixed a 

 fine copper wire; and in trying the experiment the vessel a, with the res- 

 pirable air in it, is first weighed, by suspending it from one of these copper 

 wires, in such manner as to remain entirely under water. The bottle m, 

 with the proper quantity of nitrous air in it, is then hung on in the same manner 

 to the other wire, and the weight of both together found. The air is then let 

 out of the vessel a into the bottle m, and the weight of both vessels together 

 found again, by which the diminution of bulk which they suffer on mixing is 

 known. Lastly, the bottle m is taken off, and the vessel a weighed again by 

 itself, which gives the quantity of respirable air used. It is needless to determine 

 the quantity of nitrous air by weight ; because, as the quantity used is always 

 sufficient to produce the full diminution, a small difference in the quantity makes 

 no sensible difference in the diminution. In this manner of determining the 

 quantities by weight, care should be taken to proportion the lengths of the 

 copper wires in such manner that the surface of the water in a and M shall be 

 on the same level when both have the usual quantity of air in them, as otherwise 

 some errors will arise from the air being more compressed in one than in the 

 other. This precaution indeed does not entirely take away the error, as the 

 level of the water in m is not the same after the airs are mixed as it was before ; 

 but in vessels of the same size as mine, the error thence arising can never 

 amount to the 500th part of the whole, which is not worth regarding ; and 



