Bunsen's Ice- Calorimeter. 215 



If the instrument is filled properly with pure water boiled 

 to drive out the air, and is then set up in common ice scraped 

 into an artificial snow with a chisel, the minute amount of 

 salt included in cavities is sufficient to lower the temperature 

 of the outer ice below the melting-point of the ice in the 

 instrument ; and thus, inappreciable in other ways as this 

 difference is, it causes a steady freezing of the water in the 

 calorimeter, which is quite sufficient to make accurate work 

 impossible. 



In consequence of the attention that has been given to this 

 instrument, not only by the inventor but by other physicists 

 of experience, I naturally hesitate to make any suggestion for 

 its improvement. I dare not call the slight modification 

 about to be described an improvement, but an addition is a 

 term which I may safely employ. 



The reason, as already stated, why this instrument fails to 

 work satisfactorily when used with ordinary ice is that there 

 is a slight difference of temperature between the inside and 

 the outside. In consequence of the continuous contact of the 

 outer ice or snow with the whole surface of the glass, the con- 

 ductivity for heat is very great — that is, the amount that will 

 enter or leave the instrument for 1° difference is very great. 

 The amount of heat passing through the walls is the product 

 of the difference of temperature by the conductivity. This 

 product is usually made very small by making only one factor 

 very small. It can more easily be made small by making 

 both factors small ; if one is very small, so much the better. 



This process of reasoning is so obvious that it can hardly 

 fail to have been used by some ; but I have not heard that it 

 has, nor am I aware of its ever having been put to the test of 

 experiment. I have therefore lately compared the behaviour 

 of an instrument put up in the usual way, but in common ice, 

 with the same instrument arranged to have a small conduc- 

 tivity and in the same ice. 



The plan that I have adopted to reduce the conductivity, 

 while still retaining the power to increase it to its usual ex- 

 tent at will, is to provide a protecting cover of glass in which 

 the instrument can lie, the two tubes passing through and 

 being supported by a thin indiarubber cork fixed into the 

 upper end of the glass cover. 



There is a third hole in the cork, through which is passed a 

 glass tube with a stopcock. If there is any hurry to cool the 

 instrument, ice-cold water is poured in upon the ice so as to 

 reach above the lower end of the protecting tube, and the 

 stopcock opened and the air drawn out if necessary until the 

 water reaches the cork. The water may be changed once or 

 twice by blowing through the stopcock and drawing out the 



