NON-SOLUBLE GLASS. 129 
first distillation, and 5 grams of c. p. sulphuric acid per liter before the second 
distillation. The flasks in which such water is collected or stored should be of 
resistant (non-soluble) glass and absolutely clean to begin with. With use such 
flasks or bottles become more valuable and should not be employed for other purposes. 
The solubility of glassware is best tested by determining from time to time 
the degree of electrical conductivity of pure water stored in it. The specific resist- 
ance of pure water stored for a week in such tubes, flasks, or bottles should not fall 
below 250,000 ohms. ‘The specific electrical resistance is determined upon 1 cubic 
centimeter of water exposed between electrodes having an area of 1 square centi- 
meter, and is read by means of a special Wheatstone bridge. Distilled water 
redistilled with chromic-acid cleaning mixture, and afterwards with alkaline potas- 
sium permanganate (method used by the Physical Laboratory in the Bureau of 
Soils) gives a resistance of 700,000 ohms. 
The following determinations made by the Physical Laboratory of the Bureau 
of Soils show the diverse behavior of two lots of clean test-tubes recently purchased 
as non-soluble glass by the Laboratory of Plant Pathology. 
Time of exposure, | Specific resistance, 
Kind of tube. in days. seria cieadaz 
Resistant test-tubes,(R) from Greiner 
& -ReeGhGha es wes. cess veduaoceres ro 220,000 
pe Bee ee ere ee II 219,000 
Tubes received from the School Sup- 
PLY CO erin wie sis binds v.0400. 0S cles en Be) 41,400 
DO eae Dosis ve recisic selves II 34,000 
The twice-distilled water used was taken from a Jena flask and its initial 
specific resistance was 240,000 ohms. 
MICROSCOPES. 
Microscopes of a much better grade are required for bacteriological investigations 
than for ordinary histological work. The writer has for many years employed those 
made by Carl Zeiss, of Jena, as, on the whole, most serviceable. Good microscopes 
are also made by E. Leitz, of Wetzlar, and recently by the Spencer Lens Company, 
of Buffalo, N. Y. The Zeiss stand shown in plate 15 does very well for all ordinary 
work, but is not well adapted for the making of photomicrographs or for recording 
the exact location of particular spots in the section. The latter difficulty may, 
however, be overcome by means of a removable slide-carrier attached to the stage. 
The stand may also be used with the small upright photomicrographic outfit shown 
in fig. 24 when the lens does not require a microscope having a wide tube. This 
microscope has a half-mechanical stage, an excellent fine adjustment, and good 
substage apparatus. It is thoroughly well made and very durable. One in the 
writer’s laboratory has been in use for twelve years. ‘The lacquer has disappeared 
in places and it is no longer attractive to look at, but it has required no serious 
repairs during this time and is still serviceable. 
For photomicrographic work and also for recording the exact location of desir- 
able fields in a section, the writer uses the large photomicrographic stand shown in 
plate 16. This is provided with a specially wide barrel, a fine adjustment of very 
