PROPORTION OF CHEMICALS. 35 



solves the potassium sulphate as it forms and prevents it from coating 

 the cyanic! not yet in solution. In the presence of an insufficient 

 amount of water, the potassium sulphate is not completely dissolved, 

 but forms a coating on the pieces of cyanid, preventing the sulphuric 

 acid from penetrating to it, and thereby retarding, or even in part 

 preventing, the reaction. In such cases this undissolved potassium 

 sulphate usually congeals, causing the pots to ''freeze." The phe- 

 nomenon always occurs where the formula is 1-1-1, or where the 

 same amounts of water, acid, and cyanid are used. On agitating 

 the congealed residue by stirring, it is almost always possible to find 

 small pieces of undissolved cyanid enveloped in a coating of the 

 potassium sulphate. Ordinarily, when the residue is stirred the 

 particles of cyanid are removed, to some extent, from this envelope 

 of potassium sulphate, allowing some of the unused acid to reach 

 them, and thus evolving a small amount of gas without the addition 

 of more acid. Under these conditions, however, the reaction is 

 never complete, and it is highly desirable therefore to add sufficient 

 water at the beginning to dissolve all the potassium sulphate. 



From this last statement, as well as the data presented vmder the 

 heading "The effect of too great an excess of acid" (p. 34), it is seen 

 that the congealing or "freezing" of generating jars is due to either 

 or both of two conditions: (1) An insufficient amount of water to 

 completely dissolve the sulphate of potassium, or (2) a large excess 

 of sulphuric acid, whereby the water is rendered less capable of taking 

 into solution the same amount of sulphate as it otherwise would. 



Another very important function of the water in the reaction is 

 the heat produced by the union of the sulphuric acid and water. 

 Potassium cyanid introduced into this heated mixture gives off hydro- 

 cyanic-acid gas much more quickly and thoroughly than at a lower 

 temperature, and in field work rapid generation of gas is essential. 



THE EFFECT OF DIFFERENT PROPORTIONS OF WATER ON THE TEMPERATURE OF THE GAS. 



Anyone who has watched the escaping gas and steam from the 

 reaction of potassium cyanid and sulphuric acid wherein different 

 proportions of water were used could not fail to notice that the 

 violence with which the generation starts and the gas is given off is 

 apparently greatest with the smaller proportions of w^ater. Fumi- 

 gators are aware of this, and commonly increase the proportion of 

 water when using large amounts of cyanid. Practice has demon- 

 strated that with a greater proportion of water the injurious effect of 

 the resulting gas on the leaves and fruit is materially lessened. The 

 lessening of the injury has been attributed to the fact that the escap- 

 ing gas was less heated when large proportions of water were used. 

 In order to clear up this point an experiment was performed, the 

 results of w^hich are given in Table II. 



