﻿of Phosphorus, Sulphur , and Aldehyde. 



527 



was always brought to B before a reading was made. The 

 pressure of the gas in the apparatus is then obtained by- 

 reading the difference in level between B and D : for this 

 purpose a vertical millimetre-scale was generally used, some- 

 times a cathetometer. 



C D is a barometer, the lower end of which just dips under 

 the mercury at C. The air-trap at E is useful in preventing 

 bubbles of air, which sometimes leak through the indiarubber 

 tube, from passing back into the apparatus. 



By means of the three-way tap H the apparatus can be 

 connected either with an air-pump through F, or with a little 

 vessel containing liquid aldehyde (G), which is ground to fit 

 the lower limb of the tap. The apparatus having been 



Fig. 7. 



evacuated as completely as possible, the tap is turned so that 

 the aldehyde- vapour distils into it. By repeating this opera- 

 tion several times, the air may be completely replaced by 

 aldehyde-vapour at any desired pressure up to 500-600 millim. 

 This having been done the tap H is closed, the apparatus put 

 in position in the water-bath, and the pressure of the aldehyde- 

 vapour observed. F is then connected with a tube delivering 

 pure oxygen dried over CaCl 2 , which is allowed to flow 



