A. H. Lees 247 



time. The thinning effect is very characteristic and may be explained 

 hypothetically as follows: 



When lime is slaked under water it breaks up into a great number 

 of extremely fine particles. Whether it is the fineness of the particles 

 or whether it is associated with some other property the solid matter 

 appears to be in a semi-gelatinous state. It is bulky and only sinks 

 very slowly. If allowed to dry and then re-wetted the semi-gelatinous 

 state is not re-acquired, but the particles remain gritty as though they 

 had become flocculated. If a solution of glue is added to milk of lime 

 freshly slaked the probability is that the glue precipitate so formed 

 unites the minute lime particles into larger ones, so that the mixture 

 at once loses covering power and shows the characteristic "thinning." 

 It seemed therefore profitable in view of the uncertainty of the action 

 of alkalies on glue to investigate the reaction. When 10 % solutions 

 of caustic soda were added to hot glue solutions an immediate fibrous 

 precipitate was obtained. A similar result was obtained by the addition 

 of a hot 1 % solution and an immediate slight turbidity with so weak 

 an alkali as lime water provided hot solutions were used. 



On the other hand if cold 10 % caustic soda solution was added 

 to cold glue solution no precipitate appeared for several hours. The 

 same, as was to be expected, happened if 1 % caustic soda or lime 

 water were added in the cold. The fact that the reaction between 

 alkali and glue could be slowed down opened up new possibilities for 

 lime mixtures. 



Further experimentation with milk of lime showed that thinning 

 occurred if incompletely slaked cold milk of lime or completely slaked 

 hot milk of lime were added to glue, while it did not occur if cold well- 

 slaked lime w r ere used. 



In working with milk of lime it is of course impossible to see whether 

 the gelatine of the glue is actually precipitated, but it is safe to assume 

 that loss of covering power in the mixture indicates precipitation. 

 From these facts it appeared probable that a satisfactory mixture 

 could be made by adding glue solution to cold well-slaked lime. Such 

 mixtures were made up in the laboratory and it was found that one 

 having the quantities lime 20, glue 2, water 100 gave a firm but thin 

 coat. No sudden loss of covering power was here noticed such as 

 follows when hot or unslaked lime acts on glue, but the coat was rather 

 thinner though much firmer than the control lime 20, water 100. 



This slight loss of covering power is no doubt due to the glue in 

 solution. Gelatine is used in some summer sprays in order to increase 



