1818.] M. Ha'uy on Magnetism. 117 



eye. When the coagulum is moved, it becomes granulated like 

 soft cheese ; the yellow colour is reproduced by adding a few 

 more drops of nitric acid. The acid in this case acts in the 

 same manner with the oxygen of the atmosphere, at the temper- 

 ature of from 80*5 to 95° (Far.); for the white coagulum 

 becomes yellow in two or three minutes by exposure to the sun. 

 After some hours, the yellow colour turns brown, undoubtedly 

 because the carbon is more liberated in proportion as the 

 hydrogen, with which it was combined, is burned. The coagu- 

 lum formed by the acid becomes viscid, and acquires the waxy 

 odour, which is perceived when the muscular fibre or fungi are 

 treated with nitric acid. From the interesting experiments of 

 Mr. Hatchett, it may be supposed that in this case the albumen 

 is partially converted to the state of gelatine. 



When the coagulum of the papaw is thrown into water, it 

 softens, becomes partially dissolved, and gives the water a 

 yellowish tinge ; the milk, when placed in contact with water, 

 also forms membranes ; a tremulous jelly, similar to starch, is 

 immediately precipitated, and the appearance is more remark- 

 able if we employ water at the temperature of from about 100° 

 to 140° (Far.) If carbonate of soda be added to the fluid, the 

 coagulum is not formed ; but it is immediately produced by the ad- 

 dition of an acid. If we compare together the milky juices of the 

 papaw, the cow tree, and the hsevea caoutchouc, we find a strik- 

 ing resemblance between the juices which abound in caseous 

 matter, and those in which the caoutchouc predominates. 

 According to the opinion of M. Gay-Lussac, we may consider 

 the caoutchouc as analogous to the oily part or the butter of the 

 vegetable milk ; in the vegetable milk we find caseum and caout- 

 chouc ; in animal milk, caseum and butter. The albuminous and 

 the oily principles exist in different proportions in the different 

 species of annuals and milky plants ; and in the last, they are 

 frequently united to other substances which render them injurious 

 as articles of food. 



Article VIII. 



On Magnetism considered as a Method of detecting the Presence 

 of Iron in Minerals. By M. Haiiy % * 



The magnetic property of iron serves as a method of detect- 

 ing the presence of this metal, which possesses the two-fold 

 advantage of being decisive and easy of application. In some 

 cases the iron exists in a state which enables it to be directly 

 acted upon by the magnet ; but in other minerals it is either 



* Abridged from Ann. des Mines, 1817, p. 329. 



