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 pheno- 

 mena 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 m ater 

 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. 



