Milk.—­We have seen in the last chapter that milk acts most powerfully on the leaves; but whether this is due to the contained casein or albumen, I know not.  Rather large drops of milk excite so much secretion (which is very acid) that it sometimes trickles down [page 114] from the leaves, and this is likewise characteristic of chemically prepared casein.  Minute drops of milk, placed on leaves, were coagulated in about ten minutes.  Schiff denies* that the coagulation of milk by gastric juice is exclusively due to the acid which is present, but attributes it in part to the pepsin; and it seems doubtful whether with Drosera the coagulation can be wholly due to the acid, as the secretion does not commonly colour litmus paper until the tentacles have become well inflected; whereas the coagulation commences, as we have seen, in about ten minutes.  Minute drops of skimmed milk were placed on the discs of five leaves; and a large proportion of the coagulated matter or curd was dissolved in 6 hrs. and still more completely in 8 hrs.  These leaves re-expanded after two days, and the viscid fluid left on their discs was then carefully scraped off and examined.  It seemed at first sight as if all the casein had not been dissolved, for a little matter was left which appeared of a whitish colour by reflected light.  But this matter, when examined under a high power, and when compared with a minute drop of skimmed milk coagulated by acetic acid, was seen to consist exclusively of oil-globules, more or less aggregated together, with no trace of casein.  As I was not familiar with the microscopical appearance of milk, I asked Dr. Lauder Brunton to examine the slides, and he tested the globules with ether, and found that they were dissolved.  We may, therefore, conclude that the secretion quickly dissolves casein, in the state in which it exists in milk.

Chemically Prepared Casein.—­This substance, which

* ‘Leons,’ &c. tom. ii. page 151. [page 115]

is insoluble in water, is supposed by many chemists to differ from the casein of fresh milk.  I procured some, consisting of hard globules, from Messrs. Hopkins and Williams, and tried many experiments with it.  Small particles and the powder, both in a dry state and moistened with water, caused the leaves on which they were placed to be inflected very slowly, generally not until two days had elapsed.  Other particles, wetted with weak hydrochloric acid (one part to 437 of water) acted in a single day, as did some casein freshly prepared for me by Dr. Moore.  The tentacles commonly remained inflected for from seven to nine days; and during the whole of this time the secretion was strongly acid.  Even on the eleventh day some secretion left on the disc of a fully re-expanded leaf was strongly acid.  The acid seems to be secreted quickly, for in one case the secretion from the discal glands, on which a little powdered casein had been strewed, coloured litmus paper, before any of the exterior tentacles were inflected.