ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
413 
in the usual way between slide and cover-glass often, after a short time, 
begin to deteriorate so that the sharpness of outline is lost, and the 
object is sometimes completely ruined. 
The result of the author’s observations is to show that this deteriora- 
tion is due to the effect upon the object of the material of the slide and 
cover-glass. 
The ordinary glasses used in microscopic work vary in their 
behaviour after long exposure to the air: while some retain their bright 
surface lustre others of inferior quality gradually lose it and become 
coated with a moist or dusty deposit. The same phenomena are 
exhibited by ordinary glass articles (mirrors or window panes) of 
different qualities. 
This deposit has a strong alkaline reaction, and, when formed on 
cover-glass or slide, has a considerable injurious action upon delicate 
objects mounted between them. 
Now as regards the slides, in their production a soft glass which is 
almost perfectly colourless is often used. The components of this glass 
are pure alkalies and calcium carbonate with sand free from iron. The 
amount of calcium is limited as much as possible on account of the 
fusibility. Such a glass is known in German commerce as “ Salinglas.” 
It is peculiarly liable to the deposit above described, which is sometimes 
strongly developed even in process of transport. 
Analysis of such slides by the author gave the following numbers : — 
Si0 2 
A1 2 0 3 
CaO 
K 2 0 
Na 2 0 
73*06 
0*90 
8*47 
3*87 
13*70 
100*00 
This gives the molecular ratio 
SiO, : CaO : {£° 0 
8*2 1 1*74. 
According to the author’s previous experiments, such a glass is less 
hard, and therefore less to be recommended than glasses in which the 
molecular ratios are 
6 - 7 : 1 : 1 - 1*3. 
Besides these colourless slides, others which have a slightly green 
tint are used, and these are much less liable to show a deposit. They 
are made of a window glass, rich in calcium and poor in alkalies, which 
is much more difficultly fusible. 
Cover- glasses exhibit the same differences as slides, some remaining 
unchanged by exposure while others lose their lustre. Those of English 
manufacture are the best, are less liable to form a deposit, and are also 
distinguished by uniformity in the strength, evenness and purity of the 
material. They are generally of a slightly greenish-blue tint. The 
cause of the different behaviour of the English and other cover-glasses 
