SULPHUR. 



175 



Fig. 77. 



do no harm, for it opens at once the valve, s, and the gases 

 escape. This process of raising a solid in vapor, and then 

 condensing it in the form of a powder, is called sublimation. 

 237. Properties of Sulphur. Sulphur is familiar to you as 

 a brittle yellow solid. It is insoluble in water and alcohol. 

 It takes fire readily, or, in other words, its attraction for 

 oxygen is such that it requires but little heat comparatively 

 to render its union with oxygen sufficiently rapid to occa- 

 sion the phenomena of combustion. For this reason it has 

 been much used for kindling purposes. By means of it 

 other substances that unite less readily with oxygen may 

 be heated to the degree of temperature requisite to set them 

 on fire. The kindling of a coal fire in the times of the old- 

 fashioned tinder-box illustrates well the different degrees 

 of combustibility in various substances. The iron spark 

 cast off by the blow of the flint sets fire to the finely divided 

 charcoal of the tinder ; this kindles the sulphur on the match, 



