ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 1059 



glass tube 60 cm. high, with a diameter of 2 cm., in which the steam can 

 condense, and flow back into the water-bath. 



The quantity of water in the bath is shown by the gauge /. Two of 

 the special advantages of this form of water-bath are the prevention of dust, 

 and the current of air which carries off the various vapours so that the 

 lid always remains bright, and the progress of the preparation may be 

 watched. 



New Application of the Plasmolytic Method.* — Herr H. de Vries 

 suggests an application of the method of plasmolysis for the determina- 

 tion of the molecular weight of a given substance. The calculation of the 

 isotonic coefficient of any compound soluble in water, by means of the 

 law implied in De Vries's j method for the analysis of the force of tur- 

 gidity, presupposes a knowledge of the molecular weight or equivalent 

 of the substance in question. If, therefore, the isotonic coefficient is 

 known, it follows from the law that the molecular weight can be ascertained. 

 If two substances have the same isotonic coefficients, this must result from 

 their solutions containing the same number of molecules in a given quantity 

 of water. Application of this law was made in the case of raffinose, a 

 sugar of considerable importance in the manufacture of beet-root sugar, 

 with a much higher power of rotation than cane-sugar, and affecting 

 the estimation of the latter in molasses. 



Three formulae have been proposed for raffinose, agreeing in their 

 percentage composition, viz. C12H22OU + ^HgO, C18HS2O16 + SHgO, 

 and CaeHj^Oss +10 H2O. By the application of the proposed method, 

 De Vries found the degree of concentration of raffinose isotonic with • 1 

 molecule of cane-sugar to be 5 " 957 per cent. It follows that the mole- 

 cular equivalent of raffinose must be approximately 595*7, which agrees 

 very nearly with the second of the above formulae. 



New Method for Demonstrating and Counting Bacteria and Fungi 

 Spores in the air.| — Dr. E. J. Petri's method consists in drawing air by 

 means of an air-pump through a sand filter. The sand consists of 

 particles 0* 25-0 "5 mm. and must be thoroughly heated. It is then 

 made up into the shape of corks with wire gauze. Two of these filters, 

 each 3 cm. long and 1 • 5-1 • 8 cm. broad, are inserted in a glass tube 

 8-9 cm. long. The two filters touch in the middle of the tube. The 

 second filter serves to control the efficiency of the first, and should 

 remain quite free from germs, all of which should have been picked up 

 by the first. After the filters are fitted in, the ends of the tube are 

 plugged with cotton- wool. During an experiment the plugs are removed 

 and one end of the tube connected with an aspirator. The air should 

 be removed at the rate of about 10 litres in 1 to 2 minutes. The 

 rapidity of the air stream in the filter should never exceed ■ 7 m. a 

 second. The germ-laden sand is then strewn in flat double capsules 

 about 9 cm. broad, and then liquid gelatin poured over it so as to form 

 a layer, care being taken that the sand is tmiformly distributed. As 

 the colonies grow they can be counted and examined microscopically. 

 For the purposes of examination the author has constructed a special 

 enumerator, for information about which the original must be consulted. 

 For the purposes for which it is intended, namely, the examination of 



* Bot. Ztg., xlvi. (1888) pp. 393-7. f See this Journal, 1885, p. 84. 



X Zeitschr. f. Hygiene, iii. (1887) p. 1. 



