710 SUMMARY OF CURRENT RESEARCHES RELATING TO 



this to act for one or two minutes the slide is rapidly washed in 95 p.c 

 alcohol and then transferred to solution (1) for not more than one minute. 

 It is then treated for 2-5 seconds with 95 p.c. alcohol and afterwards 

 with absolute alcohol for about one minute. The sections are treated 

 with xylol for about five minues and afterwards embedded in balsam. 

 The tracheids are yellow, the areola green, and the torus red. 



(6) Miscellaneous. 



Enumeration of Bacteria in Milk.* — R. S. Breed determines the 

 number of bacteria in milk by direct microscopical examination. The 

 sample of milk to be examined is shaken thoroughly and 0*01 c.cm. is 

 withdrawn by means of a specially constructed pipette. The milk so 

 obtained is spread evenly over an area of 1 sq. cm. on an ordinary glass 

 slide. These areas may be easily determined by placing the glass slide 

 over paper or glass on which areas of this size have been accurately ruled 

 out. The milk is then dried with gentle heat, the fat dissolved out with 

 xylol or other fat solvent, the smear again dried, then fixed with alcohol, 

 again dried and stained with some anilin dye. Alkaline or other solu- 

 tions which attack casein and loosen the smear must be avoided. The 

 counting of the bacteria is done with a Microscope and an oil-immersion 

 objective. If the diameter of the field be so adjusted by means of the 

 draw-tube that it equals 0*16 mm., then each field of the Microscope 

 covers approximately one five-thousandth (0*0002) of a square centi- 

 metre. On this basis each bacterium seen in a field taken at random 

 represents 500,000 per c.cm. if they are evenly distributed. But as it is 

 impossible to distribute them evenly, at least 100 fields should be counted. 

 The total number of bacteria seen in 10 fields multiplied by 50,000, or 

 the total number in 100 fields multiplied by 5,000, gives the total 

 number of bacteria per c.cm. Though certain objections may be raised 

 against this method, the author claims that it is more accurate than 

 the plate method. 



Metallography, etc. 



New Critical Point in Copper-zinc Alloys, f — H. C. H. Car- 

 penter and C. A. Edwards have confirmed the existence of a thermal 

 change at about 470° C. in copper-zinc alloys containing 40 to Q'S p.c. 

 copper ; this is the range in which the ft constituent is present. It is 

 suggested that this critical point corresponds to the decomposition, on 

 cooling, of /3 into a + y. In alloys which above 470° C. consisted 

 wholly of /3, the presence of a and y has been detected microscopically, 

 at high magnifications only. The equilibrium diagram, modified in 

 accordance with the authors' conclusions, is given. As y is a brittle 

 substance, the decomposition of /3 into u -j- y causes embrittling of 

 the alloy. 



C. A. Edwards discusses the nature of solid solutions, in an appendix 

 to the above paper, and concludes that (1) so-called metallic sofid solu- 



* Centralbl. Bakt., 2'e Abt., xxx. (1911) pp. 337-40 (1 fig.). 

 + Journ. lust. Metals, v. (1911) pp. 127-93 (26 figs.). 



