190 SUMMARY OF CURRENT RESEARCHES RELATING TO 



according bo circumstances. We were then able to judge of the diffusi 

 bility and activity of the antiseptic by observing the growth or absence 

 of growth of the bacteria which we had planted. Now, a comparative 



test is only of value if all the conditions are exactly the same, and I 

 think we have ultimately worked out a satisfactory method. We always 

 use the same quantity of the paste by weight, either .', gnu. or 1 grin, 

 as we wish. This is placed on an ordinary microscopical cover-glass, 

 either § or 1 in. in diameter, which is applied to the centre of the under- 

 surface of the slab of agar. In this way the antiseptic is applied to the 

 same definite area (f or 1 in.) of the agar in all cases. Where fluids 

 have been tested they have been put into a small paraffin cell containing 

 pieces of filter-paper, and always in definite quantities. 



"The slabs of agar must also always lie of exactly the same thick- 

 ness, and here we had our greatest difficulty. We began by pouring the 

 agar into a Petri dish, till, as far as we could judge, we had got the 

 proper depth of agar, and then allowed it to solidify and turned it out 

 into another Petri dish, in the centre of which the paste was laid. After 

 all. however, this was only guesswork ; the table might not be level, and 

 one side of the agar might be thicker than another, and besides we 

 could not always be certain that we had put the same amount of agar 

 into each dish. This difficulty has been overcome in a very ingenious 

 manner, and though when two or more men work together it is not 

 usual to refer to any one man's share in particular, still in this instance 

 the arrangement is likely to be very useful in similar experiments in 

 future, and therefore I think I ought to say that it was devised by 

 Edmunds, and I shall speak of it as Edmunds's cell. 



' To make an Edmunds's cell you take two square pieces of glass, 

 a brass ring of known thickness (we generally have used one \ inch 

 thick), the ring being incomplete in one part, and two or three broad 

 paper-clips. First sterilize a glass plate in the flame and then lay it 

 down on a dish, then similarly flame the interior of the brass ring and 

 lay it down on the glass, then flame the other piece of glass and lay it 

 over the brass. Bind these together by the paper-clips and you have 

 a cell with an opening at one part through which the melted agar can 

 be poured and left to solidify. When the agar has solidified the cell is 

 laid down flat, the clips removed, the upper glass plate and the brass 

 ring lifted off, and then we have the slab of agar lying on the lower 

 glass plate. The cover-glass with the paste is now placed on the centre 

 of this slab, with the paste next the agar, and then the lower part of a 

 Petri dish is inverted over it, the whole turned upside down, and with a 

 little manipulation the slab is transferred to the dish. A thin emulsion 

 of the bacteria to lie employed is previously made and is now brushed 

 over the whole surface with a camel's-hair brush. Finally, a little fluid 

 agar is run round the edge of the slab, partly to fix it to the dish and 

 partly to prevent the escape of vapour should' the antiseptic to be tested 

 be volatile. 



" As regards bacteria, we have chiefly employed the ordinary pus 

 organisms, the Staphylococcus pyogenes (///reus, but we have used 

 Micrococcus prodigiosus and also Bacillus subtilis so as to study the 

 effect on spores. It will be very interesting, when we have time, to study 



