NUTRITION IN THE INFUSORIA. 83 



closed fresh scenes of interest and of wonder in 

 microscopic worlds, peopled with hosts of animated 

 beings, almost infinite in number as in minuteness. 

 In endeavouring to render the digestive organs of 

 the infusoria more conspicuous, he hit upon the 

 fortunate expedient of supplying them with coloured 

 food, which might communicate its tinge to the 

 cavities into which it passed, and exhibit their 

 situation and course. Obvious as this method may 

 appear, it was not till after a labour of ten years 

 that Ehrenberg succeeded in discovering the fittest 

 substances, and in applying them in the manner 

 best suited to exhibit the phenomena satisfactorily. 

 We have already seen that Trembley had adopted 

 the same plan for the elucidation of the structure of 

 the hydra. Gleichen also had made similar at- 

 tempts with regard to the infusoria ; but, in conse- 

 quence of his having employed metallic or earthy 

 colouring materials, which acted as poisons, instead 

 of those which might serve as food, he failed in his 

 endeavours. Equally unsuccessful were the trials 

 made by Ehrenberg with the indigo and gum-lac 

 of commerce, which are always contaminated with 

 a certain quantity of white lead, a substance highly 

 deleterious to all animals ; but, at length, by em- 

 ploying an indigo which was quite pure, he suc- 

 ceeded perfectly.*' The moment a minute particle 



* The colouring matters proper for these experiments are such as do 

 not chemically combine with water, but yet are capable of being diffused 

 in a state of very minute division. Indigo, sap green, and carmine, 

 answer these conditions, and being also easily recognised under the 

 microscope, are well adapted for these observations. Great care 

 should be taken, however, that the substance employed is free from 

 all admixture of lead, or otlier metallic impurity. 



