OF CUBICAL p:xpan.sion of ice. 
467 
iron box of special construction, having a capacity of about 2 litres, and rectangular 
in shape, was nearly filled by means of a regular block of new pond ice, and tlie rest 
of its interior was filled up with mercury. From the weighings of this mercury and 
the ice, and a knowledge of the volume of the box, the density of the ice at 0° C. was 
com 2 :)uted. The ice contained a small cpiantity of air, the amount of which was 
separately determined and allowed for. The final value for the density came out at 
•91760. 
Nichols next turned his attention to the determination of the linear coefficient of 
expansion of ice. No work had been done on the dilatation of ice since 1852. The 
method employed (‘ Physic. Review,’ vol. 8, p. 184, 1899) w^as similar to that used by 
Struve in 1845. A bar of commercial artificial ice, which had been manufactured 
some months jirevionsly, was used, and Nichols again had the felicity of working in 
a laboratory which was never warmer than — 3° C. during the work. The readings 
were obtained l)y measuring, by means of a dividing engine, the distance between the 
centres of two tiny drops of mercury resting in dejiressions in the ice about 40 centims. 
apart. The range of temperature was from —8° to —12° C. Four sets of readings 
were taken, with the mean result '0000540 for the linear coefficient. 
Nichols’s Theory. 
In order to explain the remarkahle discre})ancies lietween the values oldained l)y 
previous observers, Nichols put forward the theory that in reality there are two 
kinds of ice which have been under experiment; the density of artificial ice being 
about '916, and that of natural ice more than one part in a thousand greater. This 
immediately throws the snlyect into a more tangible form, but the serious conse¬ 
quences of such a dual chameter for ice demand most carefid consideration. It seems 
to me that if there are really two kinds of ice, differing in density so largely, these 
varieties would also have different latent heats, and, what is perhaps more inq)ortant 
still, different melting points. 
Nichols’s theory is, however, supported entirely by his own work, and also by 
most of the results of previous observers. In this connection it must be pointed out 
that, according to one of Nichols’s ex^ieriments, natural ice assumes a density 
approaching to that of artificial ice if the natural ice has been kept some time. 
The value obtained for the density by Dueour for artificial ice is larger than that 
of other observers using the same variety. But the method of neutral equilibrium is 
far inferior in exactitude to the methods employed by Plucker and Geissler, 
Bunsen, and Nichols. Neither must the results of Bunsen be accepted as being of 
extraordinary reliability in sjiite of his assurance that lie had eliminated all error. 
The U-tube dilatometer suffers from the disability that any small difference in the 
method of holding it may cause considerable change in its voluminal contents. It 
must also be remembered that the ice in Bunsen’s experiment was probably under 
3 0 2 
