120 
DR. P. F. FRANKLAND ON THE QUANTITATIVE ESTIMATION OF 
or cotton wool, and then transferring the latter to nutrient gelatine. In consequence 
of numerous difficulties with which I was then met, I abandoned this method for 
Hesse’s apparatus, and it was not until I became convinced of the imperfections in 
the latter, to which I have referred above, that I again endeavoured to elaborate a 
new process. 
I. Gelatine Plate Method. —In the first instance, a known volume of air was drawn 
by means of a hand-pump through a small sterilised glass tube, about 4 inches long 
and ^ inch in diameter; this glass tube was fitted with a plug constructed of either a 
stratum of fine glass-powder between two layers of glass-wool or a stratum of fine 
sugar-powder between two layers of glass wool, which had been soaked in a strong 
solution of cane-sugar and subsequently dried. The tube was slightly constricted 
behind the plug, so as to prevent the latter being drawn through by the suction of 
the pump. This plug, after use, was pushed by means of a sterilised wire into a 
small sterilised stoppered bottle, and a definite volume (5, 10, or 20 c.c.) of sterilised 
water or broth added, and the whole violently shaken for some 5 or 10 minutes until 
the plug had become thoroughly disintegrated. In the case of the sugar plugs, this 
was rapidly accomplished, owing to the greater part passing into solution and leaving 
only the glass-wool skeleton behind in a fine state of division. The glass plugs are 
somewhat more difficult to break up. 
Definite quantities of the liquid were now taken out by means of a sterilised 
pipette, and added to peptone-gelatine, with which plates were poured in the ordinary 
way. The colonies subsequently making their appearance on these plates were 
counted as in the examination of water, and thus the number of organisms present 
on the whole plug was calculated. The following details of some experiments will 
serve to illustrate the method of procedure :— 
I. Science Schools, South Kensington Museum. Laboratory window (west front, 3rd 
floor). July 13, 1886. Wind, S.W.; rain on previous day. 
Volume of air aspirated = 132 litres. 
Construction of plug (single).—Glass-powder between two layers of glass-wool. 
Plug mixed with 20 c.c. of sterilised water, 1 c.c. of which gave on plate- 
cultivation :— 
Colonies, 61, 60, 47, 41 . Average = 52'25 colonies. 
132 litres of air yielded 52’25 X 20 = 1045 „ 
10 „ „ 79 
II. Laboratory window (same). July 14, 1886. Wind, W. by N.W.; strong, much 
dust blowing in street below. 
Volume of air aspirated = 106 litres. 
Construction of plug (single).—Glass-powder between two layers of glass-wool. 
Plug mixed with 20 c.c. of sterilised water, 1 c.c. of which gave on plate- 
cultivation :— 
