374 
Notes . 
substances. The theory thus established not only enables one to 
explain the known facts, but also to predict unknown ones. 
It is easy to demonstrate that any modification of iron which 
lessens its capacity of rusting at the same time diminishes its 
attraction on Phycomyces ; polished steel scarcely attracts, and 
nickeled steel does not do so at all. 
China clay, which is very hygroscopic, attracts energetically, but 
china exhibits no attraction. One of the most striking instances is 
that of agate and rock-crystal. Although both are essentially formed 
of silica, the Japanese physicist Jhmori 1 has shown that the former 
is very hygroscopic, whereas the latter is not so. And, as was to 
be foreseen, agate strongly attracts the Phycomyces, though rock- 
crystal is perfectly inactive. I might quote many similar facts, 
if necessary. Thus, sulphuric acid, sulphate of copper, &c., are 
strongly attractive. Certain bodies which are only moderately 
hygroscopic, as white soap, lose or gain moisture according to 
the degree of dampness of the surrounding atmosphere ; and in 
the first case they repel Phycomyces, in the second they attract. 
The sensibility of Phycomyces is, in fact, so great that it may 
actually be used as a reagent to test the existence of hygroscopic 
power. Having noticed that camphor very distinctly attracts the 
filaments and thymol does not (although both of these substances 
have a deleterious action on them), I was led to anticipate that 
camphor is hygroscopic — a fact which, though unknown to chemists, 
was confirmed by careful weighing. 
Lastly, the theory may be tested in another way. Unlike the 
filaments of Phycomyces, the roots of higher plants are positively 
hydrotropic. Then, as might be expected, they bend away from 
iron instead of being attracted by it. 
All these experiments succeed also in a saturated atmosphere, 
which shows that hydrotropism is not due, as generally admitted, to 
differences in the hygrometric state of the air. But, the discussion 
of this point, as well as certain deductions relative to the physical 
phenomenon of hygroscopicity, must be reserved for a detailed paper 
on this subject. 
To sum up the general results : the apparently mysterious action 
of iron on Phycomyces is nothing but a matter of hydrotropism ; 
and hydrotropism itself (negative or positive) is the bending of a 
1 Wiedemann’s Annalen , 1887. 
