4 <;6 THE POPULAR SCIENCE MONTHLY. 



right angles to a luminous beam passing among these particles in the 

 fluid "they discharge perfectly polarized light. . . . The optical deport- 

 ment of the floating matter of the air proves it to be composed, in 

 part, of particles of this excessively minute character," and it is among 

 the finest of these ultra-microscopical particles that Prof. Tyndall 

 finds the sources of bacterial life. It is almost impossible to conceive 

 a nearer approach to certainty concerning the nature of these minute 

 particles than this. Their minuteness, their capability of being physi- 

 cally demonstrated, the absolute necessity of their presence to the 

 origination of bacteria in sterilized infusions of any and every kind, 

 taken in connection with what we know concerning the germs of the 

 heteromita whose life-histories have been studied, render it simply inev- 

 itable that we have at length reached, what we are justified in believ- 

 ing to be, a genetic product of the bacteria through which their con- 

 tinuation as organisms is preserved. When first I saw the simplicity 

 and beauty of this method, it struck me that its applicability as a test 

 in reference to germs known to be such would have considerable 

 collateral weight ; and a method of employing it was suggested by a 

 fact in past experience. 1 I had in my possession a maceration of cod's 

 head, which I had kept in use for eleven months. It had become a 

 pulpy mass, and in the middle of January last it was comparatively 

 free from bacteria, but swarmed with two monads the fourth and 

 sixth of the series described by my colleague and myself. To ascer- 

 tain their exact condition, I watched them on the " continuous stage " 

 for three consecutive days, and found that both forms were to be seen 

 plentifully emitting spores. The maceration had become very short 

 of moisture, which served my purpose. I subjected it to a drier air 

 with a higher temperature, and it was not very long in becoming a 

 moist pulpy mass, with sufficient cohesiveness to be removed from the 

 vessel ; and in this condition it was placed in a heating-chamber, 

 which was slowly raised to a temperature of 150 Fahr., and kept at 

 this for an hour. This was 10 Fahr. higher than Dr. Drysdale and 

 myself had proved necessary to destroy absolutely every adult form. 

 The baked mass now appeared cracked, porous, and flaky. In parts 

 it was extremely friable, and with little pressure crumbled into almost 

 impalpable powder; while by friction a very large proportion was 

 reduced to the finest dust. To avoid all possibility of error this pow- 

 der was again exposed in the heating-chamber, spread over a plate of 

 glass, to a temperature of 140 Fahr. for ten minutes thus rendering 

 the plea of mere desiccation impossible. 



A chamber or box was now prepared precisely like Prof. Tyndall's, 

 except that there were no tubes to communicate with the outer air. 



In the " Researches " on the life-history of monads we had proved 

 that they could live, thrive, and multiply, almost as well in Cohn's 

 " nutritive fluid " as in the normal animal infusion. This fluid is com- 



1 Vide Monthly Microscopical Journal, vol. xii., pp. 262, 2fi3. 



