94 THE VITAL FUNCTIONS. 



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 attempts with 

 regard to the infusoria; but, in consequence of 

 his having employed metallic or earthy colour- 

 ing 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 sub- 

 stance highly deleterious to all animals ; but, at 

 length, by employing an indigo which was quite 

 pure, he succeeded perfectly.* The moment a 

 minute particle of a highly attenuated solution 

 of this substance is applied to a drop of water 

 in which are some pedunculated vorticellae, oc- 

 cupying the field of the microscope, the most 



* 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 other metallic impurity. 



