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 i cubic 

 centimeter of water exposed between electrodes having an area of i 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. 



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 1 6. This is provided with a specially wide barrel, a fine adjustment of very 



