PERCOLATION THROUGH GUM LAC, ETC. SLOW MOTION IN THE PARTS OF SOLIDS. 15 



under like circumstances, the smallness of such a pore is no bar to its receiving and 

 transmitting foreign atoms, but very often, in an experimental point of view, is the most 

 favourable condition under which we can study its action without any retarding or 

 complex causes. 



40. There are some experimental illustrations of the fact, that closeness of texture is no 

 hinderance to the passage of suitable bodies. I took a narrow glass pipe, about an eighth 

 of an inch in diameter, and dipping one end of it into melted gum lac, expanded there 

 on a bubble of that substance, by blowing at the other extremity. In this way, after a 

 few trials, the bubble may be made so thin as to be translucent. Such a bubble, with 

 air from the lungs in its interior, being exposed to an atmosphere of ammoniacal gas, 

 allows a free passage to it. A singular change in the appearance of the thin membranous 

 bag takes place during the experiment: from being brown in the thicker parts and whitish 

 in those that are more translucent, it becomes of one uniform flesh colour. Now in this 

 state it may be regarded as one of the most impervious of all resinous bodies, and cer- 

 tainly of them all it has the closest texture ; yet, after it has thus been exposed for 

 a short time to ammonia, we find, on passing into its interior a little reddened litmus 

 water, that the gas is present in large quantity, and must, of course, have been trans- 

 mitted along the pores in the resin. 



41. On the top of a tube which contained atmospheric air and a piece of litmus pa- 

 per, tinged red by the fumes of muriatic acid, I fastened very carefully a piece of gold 

 leaf, two tenths of an inch in diameter, with gum-water, and suffered it to dry. The 

 gold leaf, when examined with a lens by transmitted light, appeared all over of a uni- 

 form pea-green colour, nor could any hole or flaw be perceived in it. It was covered with 

 a jar of ammonia on the mercurial trough, the level of the mercury on the inside and the 

 outside being regulated. The gas went through the gold leaf rapidly, and in a very 

 short time the test paper became uniformly blue. On using carbonic acid or sulphu- 

 retted hydrogen, the action was very nearly as instantaneous. 



42. I split with a lancet a thin plate of Siberian mica, which for the most part ap- 

 peared of a flame colour, but in places where it was unequally thick, a blue or a red. 

 This plate, when substituted for gold leaf in the last experiment, suffered ammonia to pass 

 through it. 'A similar plate of sulphate of lime suffered half a cubic inch of carbonic 

 acid to pass through it in forty minutes. Atmospheric air, in all these cases, was on the 

 other side. 



43. These permeations, which we have noticed to take place so rapidly under favour- 

 able circumstances, occur likewise more slowly in nature. A sea-shell, for instance, de- 

 posited in that formation called London clay, in course of time loses its coagulated albu- 

 men, then its carbonate of lime, and its other ingredients simultaneously or successively. 

 These are replaced by the sulphuret of iron, by alumina, oxide of iron, Sec., which form 

 together a mass of so close a texture, that it can give sparks by collision. Under such 

 circumstances as those which occur along the coast of the Island of Sheppey, a thin 

 plate of carbonate of lime is permeated readily by bisulphuret of iron, so that there is a 

 continued deposition and accumulation of that substance, even in the interior of a thin 

 shell. Hence the production of that immense quantity of fossil shells, which is there 



