ON PURE AIR 191 



will at once appear, from the quantity of corrupted air 

 remaining, how much pure air was contained in it before. 

 Though there is a variety of inflammable substances and 

 mixtures fit for this purpose, I however found a mixture of 

 iron filings and sulphur the most serviceable. 



I reduced 1 Ib. of sulphur to very fine powder, and 

 mixed it with 2 Ib. of iron filings, which were not rusty, 

 moistening the whole with a little water. This mixture I 

 immediately afterwards put into several small flasks, which 

 I corked very exactly, 1 taking care at the same time not to 

 press the powder too hard into the flask ; for, in twelve 

 hours, in which time the union of iron and sulphur takes 

 place, as the black colour which it assumes shows, the space 

 occupied by this compound is greatly increased ; the flask, 

 therefore, would burst if it were pressed too hard. I shall 

 now take the liberty to describe my apparatus. 



In a pedestal of lead (&) placed in the middle of a vessel 

 (a), I fixed a glass tube ; upon the top of the tube was 

 fastened a flat piece of wood, such as would support a small 

 vessel filled with the above-mentioned mixture of iron and 

 sulphur. This apparatus was covered with an inverted 

 glass cylinder (d) t and the vessel (a) was filled with water. 2 

 When the cold was so intense as to freeze the water, I made 

 use of brandy. The glass cylinder (d) was capable of con- 

 taining 34 oz. of water. The little vessel (c), with its 

 mixture of iron and sulphur, and its stand, occupied the 

 space of 1 oz. of water. There remained, therefore, room 

 for 33 oz. 



1 The phials must be frequently opened, and if one contained the 

 whole mixture, it would be soon spoiled by the fresh air that is 

 admitted. 



2 The original essay is accompanied with a figure of this apparatus ; 

 but such assistance is perfectly unnecessary, as no one can be at a 

 moment's loss to comprehend so very simple a contrivance. T. 



