ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
481 
on the float the pressure at which the gas is delivered may be varied 
at will. The outer vessel is to be filled with water to the depth of 5 in. 
(13 cm.) ; or glycerin or liquid paraffin may be used. 
The operation of the apparatus is as follows. One of the two exit 
tubes may be connected with a manometer, or both may go to burners. 
Gas enters through the stopcock into the float which, as it rises, closes 
the valve. If the outlets be closed, the float will rise until the valve is 
entirely closed, in which position it will stand. When the exit tubes 
are opened the float falls, the valve reopens, and gas is admitted at the 
same rate at which it is consumed. The apparatus, which is made of 
metal, is designed to be used in connection with a thermostat, and with 
it the temperature may be held constant within 0*1° C. 
Paraffin Imbedding Table.* — Mr. H. B. Ward has devised a modi- 
fication of the ordinary paraffin imbedding table, which is superior to 
the older type in several respects. The table is made of a triangle of 
sheet copper with a base of 6 in. and a perpendicular height of 14 in. 
Fig. 91. 
The edges of the triangle are turned under and inward, giving to the 
table a slightly rounded margin. In height, the main part of the table 
measures 2 in., and it is about 4 in. high under the apex of the triangle 
where the flame is applied. The legs are made of five-sixteenths copper 
rod bent as shown in the illustration, and riveted to the copper sheet. 
Circular Colonometers. — Mr. H. W. Jeffers t has constructed an 
apparatus (fig. 92) for counting the colonies of bacteria on circular 
plates. It consists of concentric zones which are divided into sections, 
each having an area of 1 sq. cm. The Petri dish can be centred upon 
this apparatus by the circles, and the area read from the line its edges 
approach. To facilitate the reading of the area of the plate, the circles 
80 and 120, whose areas are equal to 80 and 120 sq. cm. respectively, 
were drawn as dotted circles, thus making the areas, marked a and b„ 
equal to one-half a square centimetre. The colonies in several areas 
can be counted, an average taken, and the result multiplied by the 
number of square centimetres in each p>late. 
The colonometer devised by Mr. J. Weiss J (fig. 93) is made up of 
eight concentric circles and 92 sector circles. The first or centre circle 
is 1 cm. in diameter ; the second 3 cm. ; the third 5 cm. ; the fourth 
7 cm., and so on to the eighth, which is 15 cm, 
* Journ Applied Microscopy, i. (1898) pp. 88-9. 
t Tom. cit., pp. 53-4 (1 fig.;. f Tom. cit., pp. 51-5 (1 fig.). 
