76 . Mr. E. P. Perman. [Feb. 4, 



apparatus, and the latter to saturate the air passing into the 

 aspirator. 



The weights employed were compared with a Ig. platinum weight, 

 which had been standardised at the National Physical Laboratory. The 

 balances employed were tested for inequality of length of arm, but in 

 each case this was found to be negligible. 



Method of Work. 



An experiment was carried out as follows : 



The four wash-bottles were thoroughly cleaned by placing them in 

 an inverted position and blowing steam through them for half an 

 hour. This, or some similar process, is important, for if there is any 

 grease present, erratic results are obtained. The flasks were then 

 about two-thirds filled with distilled water, and placed in the 

 thermostat. 



The sulphuric acid was placed in the absorption apparatus, which 

 was placed in position, and all necessary connections made. A slow 

 stream of air was aspirated through the whole apparatus, and the two 

 pressure gauges were read. 



The absorption apparatus was weighed after standing in the balance 

 case 15 or 20 minutes. Meanwhile the tube CJ was cleared of moisture. 

 The absorption apparatus was then connected up again, the rubber junc- 

 tions coated with wax, the pressure in the aspirator adjusted to that 

 previously found, and the apparatus was left for some minutes to discover 

 any possible leak. The absence of any leak being proved, the clip P 

 and the stop-cock C were opened and the experiment begun. 



' To bring the experiment to an end, the clip P was first closed, and 

 then the stop-cock C. The pressure gauge G-> was read again (it 

 usually remained quite unaltered), and the moisture was driven from 

 the tube CJ into the absorption apparatus (by the method already 

 described), which was then removed to be weighed. 



The temperature of the aspirator and the barometer were read during 

 the experiment. 



At the lower temperatures the air was never passed through at a 

 greater rate than Ol litre per minute, while at the higher temperatures 

 the rate was not more than 0'05 per minute. The air acquires the 

 temperature of the bath and becomes saturated with aqueous vapour 

 with surprising rapidity. Special experiments were made to test this 

 point. 



Saturation of the Air. 



If air is passed through one wash-bottle only, containing water, and 

 placed in a thermostat at a temperature higher than that of the air, a 

 rapid fall of temperature takes place owing to the low temperature of 

 the air and to the cooling by evaporation of the water. Experiments 

 were made at 40 C. and at 76 C. ; a thermometer which could be 



