Manchester Memoirs, Vol. xlii. (1898), No. 0, 25 



30 tons of ice, while with the water from the town's mains, 

 at a mean temperature from February to June of 45*^ R, 

 about 10 tons would suffice, town's water was used. And 

 had it not been for the known purity of this water, the 

 research would not have been attempted. 



As affording definite assurance of the purity of the 

 water. Professor Harold B. Dixon kindly furnished the 

 result of the analysis he makes periodically for the Corpo- 

 ration from water drawn in the College. From this it 

 appears that the effect of the salts dissolved is nearly 

 negligible. 



A much more important consideration was the effect 

 of the air which is dissolved in all water, and particularly 

 distilled water. It may be taken that the water used for 

 the standard capacity of heat always contains this air, and 

 it is only in such experiments as these in which the 

 temperature of the water is raised to within one-third of 

 an atmosphere of its boiling point that this air can 

 produce any sensible effect, and it does not appear that 

 this effect has hitherto been noticed. It is, of course, well 

 known that when water is heated, before it boils, what are 

 called air-bubbles rise to the surface, but on consideration 

 it will appear that these are not altogether, nor chiefly, air- 

 bubbles, but are filled with saturated steam corresponding 

 to the temperature of the water, the function of the air 

 being merely to support the excess of pressure whatever 

 it may be. Hence the volume of these bubbles represents, . 

 in addition to the heat of the water evaporated, the latent 

 h eat of this water, and this, taking the weight of air as 0*003 

 per cent, of the water (the usual amount) and the excess 

 of pressure j^ atmosphere, the effect would be a relative 

 error of 00003. ^t was to prevent this that the back 

 pressure in the thermometer chamber was made as large 

 as practicable. In this way, if the proportion of air 

 leaving the water could be ascertained, allowance could be 



