16 NATURAL HISTORY 



first who brought the truth of this theory to the test ; for 

 having tinged some water containing animalcules with 

 carmine, he found on the second day that only some dis- 

 tinct cavities in the interior of their bodies were filled 

 with the colouring matter, evidently demonstrating the 

 existence of an alimentary structure : here, however, he 

 left the subject, and it is to Dr. Ehrenberg's fiirther in- 

 vestigation of it that we are indebted for an accurate de- 

 scription of their different forms, as given in Book III. 

 In more recent experiments it has been found advisable to 

 employ vegetable colouring substances in their pure state; 

 such, for instance, as sap green and indigo, which, toge- 

 ther with the valuable acquisition of an excellent instru- 

 ment, enabled the Doctor to contribute much to our pre- 

 viously imperfect knowledge of this branch of Natural 

 History. Before I explain the manner of Viewing these 

 creatures under the Microscope, I shall venture a few 

 remarks on the Method of Procuring them. In the selec- 

 tion of vegetable substances for infusions, such as stalks, 

 leaves, flowers, seeds of plants, &c. care must be taken that 

 there be no admixture of quinine in them, or the in- 

 tention will be frustrated. Immerse these, whatever 

 they may be, for a few days, in some clear water, when, 

 if the vessels which contain them be not agitated, a thin 

 pellicle or film will be discerned on the surface, which, 

 under the microscope, will be seen to be inhabited by 

 several descriptions of animalcules : the first produce are 

 commonly those of the simplest kind, such as the Monads. 

 In a few days more, their numbers will increase to such 



