ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
135 
from the observation that in old agar cultures large and peculiar invo- 
lution forms are common ; and in that about to be described those 
involution forms may under favourable conditions be obtained within 
24 hours, and thus become a means for rapidly identifying the plague 
microbe. The microbe is to be inoculated in agar containing 2 *5-3 *5 
per cent, of salt, and incubated at 37°. In 24, and certainly within 
48 hours, every single bacillus will be swollen up and altered so that 
they resemble spheres, spindle-shaped, or oval bodies, and occasionally 
torulae, appearances which cannot easily be mistaken for any other known 
microbe. In the case of certain bacilli having a superficial resemblance 
to those of plague, isolated involution forms recalling those of plague 
may be met with, but it will never occur that the whole culture is 
so changed. 
In carrying out the test it appears to be necessary to first culti- 
vate the microbe on ordinary agar and then transfer it to salt agar. 
The authors have also found that potassium bromide or iodide can be 
used in the place of sodium chloride in a strength of about 2 per cent., 
but this variation of the method does not seem to present any special 
advantage. 
Microscopic Study of Alloys.* — The following notice of M. Charpy’s 
important work on this subject appears in the issue of * Nature ’ for 
November 4, 1897, with the signature “ T. K. R.” 
“ The study of metals with the Microscope proceeds apace, and is 
now becoming as generally pursued among metallurgists as the deter- 
mination of melting-points has been during the last five years. Since 
the appearance of Prof. Roberts-Austen’s article on ‘ Micrographic 
Analysis ’ of iron and steel, a large amount of work has been done ; 
but most observers still devote themselves more or less exclusively to 
the study of this metal, attacking unsolved problems which seem to 
have great industrial importance. This tendency is unfortunate from 
some points of view ; for the complex constitution met with in that 
protean element makes it less easy to explain the observed appearances 
until, by work on simpler alloys, a better acquaintance with the whole 
subject has been obtained. M. Charpy is one of those who has resisted 
the temptation offered by the alloys of industry, and in a recent paper 
has given some interesting results of his investigations on binary alloys 
which are well worth re-statement. 
“ It is now fairly established that microscopic examination gives an 
immediate analysis of alloys, which is all the more valuable for differing 
in its results from chemical analysis, since these differences indicate the 
existence of definite compounds, and elucidate the structure in other 
ways. The immediate analysis is now made with the aid of a plani- 
meter, as Sauveur recommended, by which the ratio of the areas 
occupied in the microscopic field by the various constituents can be 
measured. The metal or metals forming each of these constituents can 
often be indicated by their colour, hardness, and, above all, the effects 
on them of various reagents, and thus a full account of the alloy can 
be given. 
“ In the normal type of constitution of binary alloys, crystals of one 
* ‘ Etude Microscopique des AH iagea Metalliques.’ Bull. Soc. d’Encouragement, 
ii. (1897) p. 384. 
