90 ABSOEPTION SPECTRA OF SOLUTIONS. 



the absorption of aqueous solutions of strongly hydrated salts with the 

 absorption of a layer of water equal in depth to the water in the solution 

 through which the light was passed. We then carried out similar experi- 

 ments with salts which, in the presence of water, combine with only a small 

 amount of it. In a word, we compared the absorption of light by water with 

 the absorption of an equal depth of water in aqueous solutions of strongly 

 hydrated salts, and the absorption of light by water with an equal depth of 

 water in aqueous solutions of salts which are scarcely hydrated at all. 



The nonhydrated salts with which we worked were potassium chloride, 

 ammonium chloride, and ammonium nitrate. It was neeessarj r in all of this 

 work to choose salts which themselves have little or no absorption in the 

 region in which water absorbs, i. e., in the infra-red. It was found that 

 aqueous solutions of the above-mentioned compounds showed the same 

 absorption of light as water having the same depth as the water in the solu- 

 tions in question. This is exactly what would be expected. The dissolved 

 substance and the solvent do not combine with one another to any appre- 

 ciable extent, and it would be very difficult to see how either could appre- 

 ciably affect the absorbing power of the other. 



When we turn to the strongly hydrating salts, very different relations 

 manifest themselves. The salts of this class that were studied were cal- 

 cium chloride, magnesium chloride, and aluminium sulphate. 



In the case of a 5.3 normal solution of calcium chloride, the solution is the 

 more transparent from 0.9/* nearly to 1/*. The water then becomes the more 

 transparent for a short distance. From 1.05/* to 1.2/1 the solution is the 

 more transparent, becoming as much as 25 per cent more transparent than 

 the pure water. The water becomes more transparent than the solution 

 only at and near the bottom of the "water-bands" at approximately 1/*. This 

 is what we should expect if the solute exerts a damping effect on the absorb- 

 ing power of water. When a smaller depth of the solution of calcium 

 chloride is used, the water in the region 1.25/* is more transparent than 

 the solution. From this band on to the longer wave-lengths the solution 

 becomes more transparent than the water until 1.42/* is reached, when both 

 solution and water are practically opaque. 



The results for magnesium chloride are essentially the same as those 

 obtained for calcium chloride. The main difference is that from 1.0/* to 

 1.1 /*, in the case of magnesium chloride, the water is more transparent; while 

 for calcium chloride in this region the solution is the more transparent. 

 The difference between water and the solution of magnesium chloride in this 

 region is, however, not great. For wave-lengths longer than 1.1/*, the solu- 

 tion of magnesium chloride, like the solution of calcium chloride, is more 

 transparent than the water, the difference for the two salts being of the same 

 order of magnitude. 



When a smaller depth of layer of the solution was used, the water was the 

 more transparent from 1.22/* to 1.34/*. For the longer wave-lengths the 

 solution was the more transparent. 



