Report on the Bacteriology of Water. 359 



materials are not present in the active cells of the growing filaments, 

 for it is by no means improbable that the active protoplasm contains 

 totally different compounds from the reserves in the inactive spore. 

 Some such view is rather supported by the retarding action of the 

 light on spore-formation. 



7. It must not be forgotten that, in the majority of these experi- 

 ments, the intensity of light employed is very low, and that the 

 question investigated is not whether direct sunshine kills the organism 

 (for there is no doubt on that head), but how far, and in what 

 manner, ordinary daylight is effective. 



Experiments without Glass and at known Temperatures. 



As already mentioned, I contemplated avoiding glass altogether, in 

 order to test the suspicion that some of the effects observed might be 

 due to differences in diathermancy, &c. ; this was done by employing 

 thin quartz plates for all screens, floors of the cells, and even as 

 cover-slips in some cases (though optical difficulties occurred here), 

 and metallic surfaces for reflection. 



The new apparatus took some time to get together, but I had some 

 of the flat glass bottle-screens ground out by Mr. Hilger, and fitted with 

 parallel plates of quartz, and used quartz floors to the cells, and even 

 thin quartz cover-slips where necessary ; then I replaced the mirror 

 of each of the microscopes with one silvered in front, and was now in 

 a position to repeat the foregoing . observations with matters so 

 arranged, that none of the incident light could be absorbed by glass. 



This is, perhaps, the best place to give the proofs that cultures 

 standing side by side, and treated similarly in all respect?, give 

 growth curves so closely alike that we assume that the organism 

 bahaves similarly whenever subjected to the same conditions. 



The following experiments show that the curves are alike in all 

 essential respects, within certain limits of errors of measurement, 

 when exposed to like conditions. 



Comparative Measurements under like Conditions. 



In order farther to test the method of measurement, I made two 

 cultures of spores sown in 10 per cent, gelatine and traces of broth 

 and glucose, and kept at 22 C. in the dark incubator from 9 A.M. to 

 4 P.M. Germination had begun by 1 P.M., and at 4 P.M. the cultures 

 were placed at 22 C. under microscopes, exactly alike, in diffnse 

 light, the mirrors turned away in the intervals. A control thermo- 

 meter cell was employed, and in each case a sturdy-looking filament 

 selected of exactly the same length, as nearly as I could measure. 

 Care was exercised to ensure that each scale measured evenly, and 

 the measurements were made at various intervals, as follows: 



