32 DIGESTER 



hydrogen, escaping into the atmosphere, will rise and ultimately possess the higher 

 regions ; suppositions which have been the groundwork of meteorological theories at 

 different times. 



The earliest observations we possess on this subject are those of Dr. Priestley, to 

 whom pneumatic chemistry stands so much indebted. Having repeated occasion to 

 transmit a gas through stoneware tubes, surrounded by burning fuel, ho perceived 

 that the tubes were porous, and that gas escaped outwards into the fire, while, at the 

 same time, the gases of the fire penetrated into the tube, although the gas within tho 

 tube was in a compressed state. 



' In entering upon this inquiry, I found, first : that gases diffuse into the atmosphere, 

 and into each other, with different degrees of ease and rapidity. This was observed 

 by allowing each gas to diffuse from a bottle into tho air through a narrow tube, 

 taking care, when the gas was lighter than the air, that it was allowed to escape from 

 the lower part of the vessel, and when heavier from the upper, so that it had on no 

 occasion any disposition to flow out, but was constrained to diffuse in opposition to 

 the effect of gravity. The result was, that the same volume of different gases escapes 

 in times which are exceedingly unequal, but have a relation to the specific gravity of 

 the gas. The light gases diffuse or escape most rapidly ; thus, hydrogen escapes five 

 times quicker than carbonic acid, which is twenty-two times heavier than that gas. 

 Secondly, in an intimate mixture of two gases, the most diffusive gas separates from 

 the other, and leaves in the receiver the greater proportion. Hence, by availing our- 

 selves of the tendency of mixed gases to diffuse with different degrees of rapidity, a 

 sort of mechanical separation of gases may be effected. The mixture must be allowed 

 to diffuse for a certain time into a confined gaseous or vaporous atmosphere, of such 

 a kind as may be afterwards condensed or absorbed with facility. 



4 But the nature of the process of diffusion is best illustrated when the gases com- 

 municate with each other through minute pores or apertures of insensible magnitude. 



' A singular observation belonging to this subject was made by Professor Dobe- 

 reiner, of Jena, on the escape of hydrogen gas by a fissure or crack in glass receivers. 

 Having occasion to collect large quantities of that light gas, he had accidentally made 

 use of a jar with a slight fissure in it. He was surprised that the water in the 

 pneumatic trough rose into this jar one and a half inches in twelve hours ; and that 

 after twenty-four hours the height of the water was two inches two-thirds above the 

 level of that in the trough. During the experiment, neither the height of the baro- 

 meter nor the temperature of the place had sensibly altered. He ascribed the phe- 

 nomenon to capillary action, and supposed that hydrogen only is attracted by the 

 fissures, and escapes through them on account of the extreme smallness of its atoms. 

 It is unnecessary to examine this explanation, as Dobereiner did not observe the whole 

 phenomenon. On repeating the experiment, and varying the circumstances, it ap- 

 peared to me that hydrogen never escapes outwards by the fissure without a certain 

 portion of air penetrating at the same time inwards, amounting to between one-fourtli 

 and one-fifth of the volume of the hydrogen which leaves the receiver. It was found, 

 by an instrument which admits of much greater precision than a fissured jar, that 

 when hydrogen gas communicates with the air through such a chink, tho air and 

 hydrogen exhibit a powerful disposition to exchange places with each other ; a par- 

 ticle of air, however, does not exchange with a particle of hydrogen of the same mag- 

 nitude, but of 3 '83 times its magnitude. Wo may adopt the word diffusion-volume to 

 express this diversity of disposition in gases to interchange particles, and say that tho 

 diffusion-volume of air being 1, that of hydrogen gas is 3'83. Now, every gas has a 

 diffusion-volume peculiar to itself, and depending upon its specific gravity. Of those 

 gases which are lighter than air, the diffusion-volume is greater than 1 ; and of thoso 

 which are heavier, the diffusion-volume is less than 1. The diffusion-volumes are. 

 indeed, inversely as the square root of the densities of the gases. Hence, the times of 

 the effusion and diffusion of gases follow the samo law.' See Watts's ' Dictionary of 

 Chemistry ; ' Graham's ' Elements of Chemistry ; ' and papers in Philosophical Trans- 

 actions of the Eoyal Society.' 



DIGESTER is tho namo of a kettle or pot of small dimensions, mado very 

 strong, and mounted with a safety valve in its top. Papin, tho contriver of this appa- 

 ratus, used it for subjecting bones, cartilage, &c., to the solvent, net i.n of hich-; 

 steam, or highly-heated water, whereby he proposed to facilitate their di<_ r <-Mi<m in 

 the stomach. This contrivance is tho origin of the French cookery pans, railed 

 autoclaves, because the lid is self-keyed, or becomes steam-tight by turning it r->uml 

 under clamp:; or ears at the sides, having been previously ground with rnu-ry t< tit tho 

 edge of the pot exactly. In some autoclaves the lid is merely laid on with a fillet of 

 linen as a lute, and then secured in its place by means of a PCTCW bearing down upon 

 its centre from an arch bar above. Tho safety valve is loaded cither by a weight 

 placed vertically upon it, or by a lever of the second kind pressing near its fulcrum, 



