Progress of Invention. 475 



more importance, to the purposes of the photographer ; and in such 

 a way as that, to him, the day was practically rendered twenty-four 

 hours long, and the darkest cavern became as well adapted to his 

 art as the most brilliantly lighted studio. Anew, and scarcely less 

 important use of magnesium has now been discovered. It is found 

 to precipitate metals from their solutions, and in such circumstances 

 as to afford valuable aid to the toxicologist in medico-legal inves- 

 ligations regarding cases of supposed poisoning. Sometimes it 

 gives rise to precipitation in the metallic state. Thus it throws 

 down iron and zinc from slightly acidulated solutions of their 

 proto or sesqui salts ; and cobalt from a similar solution of its pro- 

 toxide. In these, and other cases, a very remarkable circumstance 

 occurs — hydrogen is evolved. The evolution of this gas, however, 

 prevents arsenic, or antimony, from being precipitated in the 

 metallic state ; since each of these metals combines with the 

 hydrogen, and passes off with it. The iron, zinc, and cobalt thus 

 precipitated assume a brilliant metallic appearance if they are 

 well-washed, and then compressed — a very slight pressure being 

 sufficient with the zinc. Iron, cobalt, and nickel obtained in this 

 way are highly magnetic. These are not the only metals which are 

 thrown down by magnesium. It precipitates also gold, silver, pla- 

 tinum, bismuth, tin, mercury, copper, lead, cadmium, and thallium. 

 Other important circumstances are the consequence of the great 

 solubility of magnesium. Thus it decomposes water with great 

 rapidity, if a very small quantity of chloride of sodium or sal 

 ammoniac is present ; and the hydrogen thus evolved when the mag- 

 nesium contains no silicium, is quite pure. The great tendency of 

 magnesium to become oxidized also renders it highly effective as a 

 battery element. If a plate of magnesium, only OT gramme in 

 weight, is immersed, with a very small plate of copper, in a glass 

 tube which has a capacity of six cubic centimetres, and is filled with 

 Water which has been acidulated, a galvanic current will be pro- 

 duced that will suffice to keep a small electro-magnetic apparatus, 

 or to afford with a Geissler tube an illumination ten centimetres in 

 length for about ten minutes. 



New Application of Colloid Diaphragms. — A mode of dialysing 

 gases by means of colloid diaphragms has recently been discovered. 

 It has long been known that even a thin pellicle of caoutchouc is 

 totally impervious to gases, as such. But it has been found that it 

 is capable of liquifying certain gases, which then pass through it, 

 and are again restored to the gaseous form, or reaching the other 

 side of the pellicle. Atmospheric air is one of these gases ; but its 

 constituents are not transmitted with a velocity proportioned to 

 their relative amounts ; the whole of its oxygen, but only half 

 of its nitrogen is allowed to pass through. We are thus 

 enabled to obtain oxygen sufficiently pure to rekindle incandescent 

 wc od. To render the pellicle as effective as possible, the air to be 

 dialysed must have free access to one side of it, and a partial 

 vacuum must be maintained at the other. Colloid membranes are 

 not the only simple means of liquifying gases. Platinum, pal- 

 ladium, and iron condense hydrogen into a liquid which, possibly, 



