204 ANALOGOUS CHANGE IN PHOSPHORUS. 



927. Again, when speaking of phosphuretted hydrogen, he says, " Exposed to the 

 influences of the direct solar light this gas is decomposed, a part of the phosphorus 

 separates under the form of red phosphorus, and is deposited on the interior surface of 

 the glass. If we cover the vessel which contains the gas imperfectly, no phosphorus 

 is deposited on the covered spaces." (lb., torn, i., p. 265.) 



928. As BERZELIUS does not give these experiments as his own, and I do not know 

 to whom we are indebted for them, I repeated some of them. Among other corrobo- 

 rative results, it appeared that a piece of phosphorus of a pale or whitish colour, in a 

 vessel filled with pure and dry carbonic acid gas, placed in the sunshine, exhibited the 

 phenomenon in question. Eventually the phosphorus became of a deep blood-red 

 colour, and on the sides of the glass towards the light feathery crystals formed, the tint 

 of which bore a close resemblance to that of the red prussiate of potash. 



929. Since the invention of the tithonometer, I have been able to observe more 

 closely the habitudes of chlorine. In the description given of that instrument in 

 Chapter XVI., it is recommended to cast aside the first observation, because it never 

 gives an accurate estimate of the true effect. When a mixture of chlorine and hy- 

 drogen is exposed, muriatic acid does not immediately form ; but a preliminary titho- 

 nization is necessary, and then, at the end of a certain period, contraction begins to 

 take place. 



930. A tithonometer exposed to the daylight is much too powerfully affected to 

 allow of the successive stages of change to be distinctly made out ; the preliminary 

 tithonization is accomplished so rapidly, that the indications of it are merged and lost 

 in the contraction which instantly follows. It is necessary, therefore, that we should 

 operate with a small lamp-flame. 



931. To such a flame I exposed a mixture of chlorine and hydrogen, and marked 

 the number of seconds which elapsed before contraction, arising from the production 

 of muriatic acid, took place. The first indications of movement occurred at the close 

 of 600 seconds. 



The index then moved through the first degree in 480 seconds. 

 " " " second " 165 " 



" " " third " 130 " 



" " fourth " 95 " 



" " " fifth " 93 " 



" " sixth " 93 " 



and continued to move with regularity at the same rate. 



932. These observations, therefore, prove that a very large amount of radiant matter 

 is absorbed before chemical combination takes place ; and that in the case of chlorine 

 and hydrogen, the total action is divisible into two periods : the first, during which a 

 simple absorption is taking place without a chemical effect ; the second, during which 

 absorption is attended with the production of muriatic acid. 



933. The facts which I am endeavouring to set forth prominently in this communi- 

 cation are, 1st, the preliminary tithonization just discussed ; and, 2d, the persistent 

 character of the change impressed upon chlorine when it has been exposed to the sun, 

 an effect wholly unlike a calorific effect, which would soon disappear. 



