AN UNKNOWN STIMULANT. 165 



A superabundance of food was purposely supplied through- 

 out all the experiments, and all the glasses in which the food- 

 plants — Algoe, Elodea camadensis, and Lewna — did not grow 

 luxuriantly were emptied, so that from the very first, in all the 

 experiments on which the curves were founded, the injurious 

 effects of any kind of dearth of food were excluded. 



. The temperature of the water, so long as it oscUfates within 

 an insignificant range near the optimum, also has no efiect on 

 the volume-curve, or at any rate a very trifling one ; if the in- 

 fluence of volume had been affected by variations in the tempe- 

 rature of the water, all the animals in the various experiments 

 growing up in different bodies of water, so long as they were at 

 a similar temperature, must have attained the same or nearly 

 the same size. Temperature does not exercise a really decisive 

 influence till it approaches one of the two utmost endurable 

 extremes. 



It might also be supposed that the different proportions of 

 oxygenated air or carbonic acid contained in the water were the 

 efficient cause ; but this is easily disproved. In the first place, 

 it is not very easy to see how, in that case, such regular curves of 

 volume could arise, since the deficiency of air in each vessel must 

 then have been always in the same proportion as the body of 

 water; and this can scarcely be assumed as probable. On the 

 other hand, this influence was already excluded by the fact that 

 the superabundance of plants growing in the water disengaged 

 so much oxygen that the water in all the glasses must have 

 been absolutely saturated with it, and the stratum of air in 

 contact with the surface, which, as is well known, is breathed 

 by the Lymnsea, must have been equally and perfectly pure. 

 For the same reason the carbonic acid disengaged by the animals 

 must always have been entirely reabsorbed by the plants. 



The salts which can be proved to be present in water can 

 just as little be regarded as the cause of the dwarfed growth of 

 the animals. With the assistance of a chemist, my friend 

 Professor Hilger of Erlangen, I repeated my experiments with 

 distilled water, and with water which was saturated with the 

 constituents proved normally to occur in water (such as mag- 

 nesic sulphate, calcic carbonate, &c.), and the normal course 



