MEMORANDA. 61 



Plis evidence consists, on tlie one liand^ of the fact stated 

 by him, that his investigation of the atmosphere with his 

 aeroscope has not enabled him to detect the " ova of infu- 

 soria ;" and, on the other hand, that when " suitable'' infu- 

 sions are exposed to the air, millions of " infusoria" are sure 

 to make their appearance in it. 



(I would draw especial attention to the words in italics.) 



At the same time, he declares that the " ova" are " infi- 

 nitely rare," even in situations where they might be expected 

 to occur. 



In the first place, it is right that I should remind your 

 readers of the fact (of which I can hardly suppose Dr. Pouchet 

 to be ignorant), that the term "infusoria/^ formerly applied 

 by Ehrenberg and others to a great variety of forms belonging 

 to the Protophyta, Protozoa, Annuloida, &c. &c., is now 

 restricted to that group still denominated " Polygastrica,'^ by 

 Dr. Pouchet. 



As before stated, in many of these forms, conjugation of 

 the " germ" and '' sperm" cells has never been traced, and I 

 think I am correct in saying no " ova" have been discovered. 



It is therefore not surprising that Dr. Pouchet should not 

 have been able to detect the " ova" of Polygastrica (so called) 

 in the atmosphere^ granting even the utmost perfection to 

 his apparatus ; and I should be much surprised if I heard 

 that even the highest powers of our microscopes had revealed 

 the dried germs of these organisms in their earliest stage. 



This brings us to the second phase in Dr. Pouchet^s evi- 

 dence. He says, that whenever a suitable infusion is employed, 

 and placed in contact with not more than a decimetre of air, 

 millions of infusoria are almost sure to make their appearance. 



He does not state of what his " suitable infusion" consists, 

 nor what are his infusoria. 



In No. XVII (October, 1856) of this Journal, you pub- 

 lished an abstract of my paper, read before the British Asso- 

 ciation, in which I described an experiment tried by me with 

 an infusion of chlorophyll. This consisted of the juice of 

 cabbage mixed with a solution of gum, and baked at an in- 

 tense heat over a furnace, so that all traces of life must have 

 been destroyed ; the chlorophyll cake thus obtained was dis- 

 solved in distilled water, and this formed the infusion. 



I found, on exposing this compound to the air, that in a 

 day or two, a few of the forms known as " Glaucoma scintil- 

 lans" made their appearance; and these multiplied with 

 incredible rapidity. The conclusion at which I arrived from 

 this experiment was, that the dried zoospores, or germs, 

 floated about in the atmosphere ; and I had at least as good 



