CHEMICAL CHANGES. 171 



those gaseous mixtures is exposed to sunshine, they 

 combine rapidly, and often with explosion. A solution 

 of the sulphate of iron in ordinary water may be pre- 

 served for a long time in the dark without undergoing 

 any change ; expose it to the sunshine, and a precipita- 

 tion of oxide of iron is very rapidly produced. The 

 mineral chameleon, the manganesiate of potash in solu- 

 tion, is almost instantly decomposed in daylight ; but it 

 is a long time before it undergoes any change in dark- 

 ness. The same thing occurs with a combination of 

 platinum and lime : indeed, it appears that precipi- 

 tation is at all times, and under all circumstances, 

 accelerated by the solar rays. As these precipitations 

 are in exact agreement with the quantity of actinic 

 radiation to which the solutions have been exposed, we 

 may actually weigh off the relative quantities, represent- 

 ing in grains the equivalent numbers to the amount of 

 actinism which has influenced the chemical compound.* 



" If a solution of peroxalate of iron be kept in a dark place, 

 or if it be exposed to 212' of Fahr. for several hours, it does not 

 undergo any sensible change in its physical properties, nor does it 

 exhibit any phenomenon which may be considered as the result of 

 any elementary action. 



" If, however, it be exposed to the influence of solar light in 

 a glass vessel provided with a tube, the concentrated solution of 

 oxalate of iron soon presents a very interesting phenomenon : iu 

 a short time the solution receiving the solar rays, developes an 

 infinite number of bubbles of gas, which rise in the liquor with 

 increasing rapidity, and give the solution the appearance of a 

 syrup undergoing* strong fermentation. This ebullition always 

 becomes stronger, and almost tumultuous, when an unpolished 

 glass tube is immersed in it with a small piece of wood; the 

 liquid itself is afterwards thrown into ascending and descending 

 currents, becomes gradually yellowish, turbid, and eventually 

 precipitates protoxalate of iron, in the form of small brilliant- 

 crystals of a lemon-yellow colour, gas continuing to evolve." 

 Chemical action of light, and formation of Humboldtine by it', 

 Phil. Mag., 1832, second series. "When a solution of plati- 

 num in nitro-muriatic acid, in which the excess of acid has been 

 neutralized by the addition of lime, and which has been well cleared 



