MECHANICS AND USEFUL ARTS. 57 



of the stearine had been applied, the place was left until it had be- 

 come completely cold ; the surrounding earth was then dug out care- 

 fully at some little distance from the edges of the impression, and 

 the portion containing this latter was lifted up in one entire block, 

 and laid on a cloth several times doubled, the edges of which were 

 raised up so as to form a kind of border, or rather framing, into 

 which, and against the sides of the sandy earth containing the im- 

 pression, plaster of paris was poured ; and when the latter was set, 

 the whole could be handled without danger, and was firm enough 

 to bear packing and carriage to any distance. It is evident, there- 

 fore, that, if necessary, it might also be used as a mould, from 

 which casts in plaster could be obtained. The value of such a pro- 

 cess, as an aid in criminal cases, is too self-evident to require demon- 

 stration ; the production of the tell-tale impressions in a court of 

 justice, where every mark can be conveniently exhibited and com- 

 pared with the object by which it was produced, may be equally 

 useful in the proof of guilt and of innocence, and it would be 

 strange indeed if a use for such a process be not discovered in mat- 

 ters of scientific or practical interest. 



FILTRATION AND FILTRATION MEDIA. 



The following is an abstract of a paper on the above subject re- 

 cently read before the London Society of Arts, by Julius I)ahlke, 

 Esq. 



" During the past seventy years, gravel, sand, and charcoal, used 

 as a mixture, have been the agents most in vogue amongst filter 

 makers, and it is only lately that due attention has been paid to 

 charcoal as the most efficient filtering medium. Its use is much 

 more frequent now, because not only has it a powerful detergent 

 effect, but it possesses also the peculiar advantage of not becoming 

 foul, while it protects from decomposition other bodies in contact 

 with it. It has been often asked why animal charcoal is so effec- 

 tive as a filtering medium. Some attribute this to the presence 

 of so much carbon ; but that this is an insufficient reason, is shown 

 by the fact that, although coke contains more carbon than sand, 

 yet it is not superior as a filtering agent. Animal charcoal filters 

 about three and a half tunes more rapidly than either coke or sand, 

 while it is also greatly superior in this, that it removes many inor- 

 ganic impurities held in solution, over which the former substances 

 exert no power. It appears that the more porosity a filtering 

 medium possesses in itself, the more rapidly does it filter, and the 

 greater is the effect it produces on the water. The latter will be 

 still more decided when, with a greater porosity, peculiar substan- 

 ces are combined. 



" This leads me to believe that we may attribute the extraor- 

 dinary filtering quality of animal charcoal to the fact that its princi- 

 pal component parts are lime and carbon, so combined as to secure 

 a wonderfully fine porosity. Vegetable charcoal, although very 

 porous, and containing far more carbon, has less effect on water. 

 Although we know of powerful agents for the removal of different 

 impurities from water, circumstances may and do interpose which 



