216 Addition to Bunsen's Ice- Calorimeter. 



air again ; thus the instrument is quickly brought to the 

 freezing-point. The water may then be drained away, when 

 the instrument will be left in an ice-cold chamber, but not 

 in contact with anything but air, except at its upper end. 



For this arrangement the two tubes above the joint should 

 be two or three times the usual length, but an ordinary in- 

 strument can easily be adapted by prolonging these tubes at 

 their upper end. It is essential that both tubes be carried 

 through the ice much further than usual, so that no heat may 

 be conducted down to the working part, as any stray heat 

 would be more mischievous than usual, in consequence of the 

 insulation of the instrument. The upper end cut off from 

 a gas jar makes a convenient glass cover. Its projecting 

 rim should be placed above, where it serves to support the 

 whole instrument, if it is allowed to rest on a tripod-ring 

 standing in the ice-box. The index-tube that I have used 

 has a mean capacity of '0001285 cub. centim. per milli- 

 metre, very nearly as fine as that used by Bunsen. The in- 

 strument was carefully filled with the distilled water from the 

 general supply, which occasionally is found to be by no means 

 pure. From the steady melting observed in the experiments, 

 it would appear to contain some impurity. The ice used in 

 the box was ordinary Norway ice. When the instrument 

 was put up in the usual way in this ice, the movement of the 

 index showed generally a melting in the instrument, which 

 was reduced enormously when it was surrounded by its cover 

 and placed in the same ice. With the cover, in some cases, 

 there was a movement of not more than 1 millim. in about 

 three hours ; but I did not, in these cases, obtain a perfectly 

 trustworthy comparison. In one case, when the movement 

 was greater, I measured the rate with the cover, then without, 

 and then with the cover again. It showed a melting at the 

 rate of 4 millim. an hour with the cover, of 27 an hour with- 

 out the cover, of 30 an hour without the cover the next day, 

 and of 4 an hour with it on again. Thus, by means of the 

 cover the instrument was brought from a state in which the 

 unfavourable conditions made it almost unserviceable to one in 

 which it could be well used. In another case I purposely put a 

 trace of salt with the ice in the box, and found the rate of 

 freezing to be from 8 to 10 times as great without the cover 

 as with. In all these cases the instrument had been quickly 

 taken from the ice, had its cover removed, and been at once 

 replaced in the cavity left in the ice by the cover. The ice 

 was then pressed down with a stick ; but it is probable that 

 there was by no means so continuous a current as there would 

 have been had the freshly-cut ice been put in after the instru- 

 ment. Hence it is probable that the ratio of about 7 to 1 



