Heat of Formation of Alloys. 39 



to be measured, since the mercury had to be weighed in 

 another room. 



The water-values of the calorimeters were measured experi- 

 mentally. This was done by running in mercury at 100° C, 

 the operations being in all other respects precisely the same 

 as when a heat of solution was being determined. In each 

 case the mean of five to ten experiments was taken. 



The order of procedure was the following : — The mercury 

 in which the metal or alloy was to be dissolved was weighed 

 into the inner vessel of the calorimeter, and the thermometer 

 and stirrer placed in position. The alloy (or metal) was then 

 filed, and the filings weighed and introduced into the receiver 

 d. After the lapse of about a quarter of an hour the stirrer 

 was agitated (always by hand) and the temperature read at 

 short time-intervals. Sufficient readings having been ob- 

 tained for the radiation curve, the filings were allowed to drop 

 on to the mercury by lowering h, and temperature readings 

 were taken for the next 5 to 15 minutes. Solution was often 

 complete in less than a minute, and rarely took more than 2h 

 minutes. 



The mercury was always purified before use by treatment 

 with nitric acid. One-fifth of its volume of concentrated acid 

 was poured on, and the mercury thoroughly agitated. The 

 whole was left over-night, and the crust of nitrate broken off 

 and removed as far as possible in the morning. The mercury 

 was then washed with water, dried with calcium sulphate, and 

 filtered through chamois leather. Moreover, each alloy and 

 its corresponding mixture were always dissolved in mercury 

 from the same batch, so that only the heat of solution, and 

 not the heat of formation, could be affected by impurities. 

 The metals were obtained from Kaulbaum ; but some experi- 

 ments with commercially pure zinc showed that small quantities 

 of impurities did not appreciably affect the result. To make 

 the alloys, it was found best to melt the metal of which the 

 larger quantity was being used in a graphite crucible, and to 

 drop the other in solid. All the metals used melt at a low 

 temperature, and very little oxidation seems to have taken 

 place. When a eutectic * alloy was wanted, the fluid mass 

 was poured, after stirring, into a deep cylindrical mould main- 

 tained for some time a few degrees above the melting-point 

 of the eutectic and then gradually cooled down. The line of 

 demarcation between the eutectic and the upper layer was 

 invariably sharp. The alloy A, in Table I., was formed by 

 dissolving the requisite quantity of zinc in molten lead ; B 



* This is Dr, Guthrie's name for the alloy with the lowest melting- 

 point. 



