260 THE CHEMISTRY OF CREATION. 



knowledge a Divine Hand in these visitations, 

 for the remedying of which there is little help in 

 man, nor to inquire wherefore they are sent. 



From the property possessed by ozone of de- 

 composing iodide of potassium, in such a man- 

 ner as to strike a blue tint when a solution of 

 starch is added, it is to be easily detected in the 

 air. Perhaps some of our readers may feel in- 

 clined to repeat the following plan for detecting 

 it in the air. The directions are: "Slips of 

 paper are to be smeared with the following com- 

 position : a drachm of common WHITE starch is 

 mixed with an ounce of boiling water, and the 

 solution boiled until it is of the consistence of 

 that used in the laundry ; then twelve grains of 

 iodide of potassium are to be added, and the 

 whole well mixed together. The presence of 

 ozone is indicated by the decomposition of the 

 potassium salt, and the formation of a blue iodide 

 of starch." We may look forward with interest 

 to the fresh discoveries of chemistry upon the 

 uses of this occasional ingredient of the air. 



Sulphuretted hydrogen gas is one of the most 

 common occasional ingredients in the air of 

 towns. It is perceptible by its peculiar odour 

 like that of rotten eggs, and is rendered pro- 

 vokingly sensible by effects on white paint, the 

 white lead of which it decomposes and turns 

 black It is emitted from sewers and drains 

 in large quantities. A house in Paris fresh 



