HYDROSULPHURIC ACID. 215 



springs, is an effect of hydro-sulphuric acid gas. Its 

 discoloring effect may be illustrated, by pouring a little 

 dilute sulphuric acid upon a few grains of sulphuret of 

 iron, in a tea-cup, and holding a bright moist coin in the 

 fumes. Its effect on paints may be shown by exposing 

 a piece of paper, moistened with solution of sugar of 

 lead, in the same manner. The white paper immedi- 

 ately assumes a dark metallic stain. Paper moistened 

 with a solution of tartar emetic, takes a deep orange hue. 

 This experiment is often varied, by drawing amusing 

 figures on paper, with lead solution, and bringing them 

 out by exposure to the gas. 



534. EXPLANATION. The change of 



Explain the , 



cause of the color in each case, is owing to the forma- 

 toior 9e f tion of a metallic sulphide, having a diffe- 

 rent, and generally a darker color. Zinc 

 is not blackened, because its sulphide happens to be 

 white. For this reason, chemical laboratories, and other 

 places where hydrosulphuric acid is likely to be evolved, 

 should be painted with zinc paints, instead of those 

 containing lead. 



535. RELATIONS TO LIFE. Sulphuretted 



What is the 



effect of ml- hydrogen, if inhaled in any considerable 

 ?f on ow- quantity, acts as a poison. Caution should 

 therefore be observed, in experiments with 

 this gas. The mixture of gases which is given off 

 from recently ignited coal, contains sulphuretted hy- 

 drogen acid, in large proportion, and owes its deleterious 

 qualities, in considerable part, to this admixture. 



