OCR Text |
Show 114 DROSERA ROTUNDIFOLIA. CHAP. VI. f rom the 1e av es , and this is likewise characteristic. of chemically prepared casein. Minute ~rops of milk, 1 d leaves were coagulated In about t en p ace on ' . f 'lk . t Schiff denies* that the coagulation o mi minu es. h 'd h' h by gastric juice is exclusiv~ly_ due to t e aci w ~c . I·s presen t , but attributes It In part. to the pepsin, and it seems doubtful whether with Dr?sera the coagulation can be wholly due to the. acid, as the secretion does not commonly colour h tm ~s paper until the tentacles have ' become well Inflected; whereas the coagulation commences, as we hav~ seen, in about ten minutes. Minute drops of slnmmed milk were placed on the discs of five leaves; and a large proportion of the coagul~ted 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 see~ed at first sight as if all the casein ~ad not been dissolved: ~or 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 oilglobules, ~ore or less aggregated tog~~her, ~vith 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 ~issolved. We may, therefore, conclude that the secretion quickly dissolves casein, in the state in which it exists in milk. Chemically Prepared Oasein.-This substance, which * 'Le<;ons,' &c. tom. ii. p. 151. CHAP. VI. DIGESTION. 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 reexpanded 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. Small cubes of hard casein, moistened with water, were placed on two leaves; after three days one cube had its angles a little rounded, and after seven days both consisted of rounded softened masses, in the midst of much viscid and acid secretion; but it must not be inferred from this fact that the angles were dissolved, for cubes immersed in water were similarly acted on. After nine days these leaves began to reexpand, but in this and other cases the casein did not appear, as far as could be judged by the eye, much, if at all, reduced in bulk. According to Hoppe-Seyler and Lubavin* casein consists of an albuminous, with * Dr. Lauder Brunton, 'Handbook for Phys. Lab.' p. 529. I 2 |