Dr. Thomas Andrews : The Great Chemist 119 



the question of latent heat in many substances. Joseph Black 

 first brought this doctrine into use, and it must be remembered 

 that it is very closely connected with the ideas raised by the 

 investigations into heat of chemical combination. It differs by 

 being concerned only with the physical change of a substance- 

 Black showed for example, that it required a large quantity of 

 the entity called heat to cause the melting of ice, but this heat 

 could not be perceived by the thermometer, as it had disappeared 

 in the change of physical state. In like manner heat was required 

 to convert the water into steam, and again the thermometer gave 

 no indication of its presence, as the steam was at precisely the 

 same temperature as the water from which it was being formed. 

 In all such changes of state heat is either absorbed or given out ; 

 the quantities are perfectly measurable and are fixed, being always 

 the same for the same change of condition. It is also the fact 

 that the amount of heat required to raise the temperature of a 

 substance, water for example, through some range between fixed 

 limits, is always the same for the same substance under the same 

 conditions and is proportional of course to the mass of substance 

 dealt with. This is the specific heat of the substance, and the 

 unit used is generally the quantity of heat required to raise one 

 gramme through one degree C, from a fixed temperature. For 

 fifty years it was believed that Regnault was light in his hypothesis 

 that the specific heat of water was almost constant between 

 0° and lOO'C, between freezing and boiling point, and all Andrews' 

 experiments were made with this idea in view. In 1879 Rowland 

 discovered the most unexpected fact that, instead of a constant, 

 or a very slight but steady increase in the specific heat, the 

 measure at 35° was almost one per cent, less than at 5°C. In 

 the light of these results all Andrews' determinations need to be 

 recalculated, as his values aie given in terms of the mean between 

 0° and 100°, while his measurements were made at temperatures 

 at which the specific heat is now known to be above the mean. 

 I may illustrate this by his figures for water vapour. 



He published in 1847 a series of eight experiments on water 



