PROPERTIES OF FLUOR SPAR. 141 



5. Fluoride of calcium, or fluor spar, is a substance well 

 known to mineral collectors, from the beautiful cubical and 

 octohedral crystals which it forms, of various colours. To the 

 chemist it is interesting as the source of the hydro-fluoric 

 acid a substance which, from its property of corroding or 

 etching glass, and of acting powerfully upon nearly all other 

 bodies, is one of the most remarkable compounds with which we 

 are acquainted. Fluor spar is easily reduced to the state of a 

 fine, generally white powder. Though not absolutely insoluble 

 in water, it is very sparingly so at any temperature. 



In some of our lead-mining districts, it occurs in considerable 

 quantity as a mineral production ; and as sensible proportions 

 of it are contained in the teeth and bones of animals, it must be 

 present also in plants, and everywhere, in minute quantity, 

 distributed through the soil. 



6. Muriatic acid is an acid gas which is produced, and rises 

 in white vapours into the air, when sulphuric acid (oil of vitriol) 

 is poured upon common salt. It has a strong affinity for water, 

 which absorbs about 400 times its bulk of the gas. The liquid 

 acid solution thus obtained is the common muriatic acid, or 

 spirit of salt, of the shops. When the water is saturated, it 

 contains 40 per cent (40.66) of the dry acid gas ; but the 

 acid of the shops is very variable in strength, and often contains 

 as little as 20 per cent of dry acid. In the succeeding chapter, 

 when recommending experiments with this acid, I shall describe 

 a simple method of ascertaining its strength. 



2. Suggestions for comparative experiments loith the chloride 

 of potassium applied alone. 



I am not aware of any experiments having been made with 

 chloride of potassium applied alone. The refuse of certain 

 soap-works contains this salt, mixed with gypsum and various 

 other substances, and this mixture has often been applied as a 

 manure with profitable results. But such trials throw no light 

 upon the special action of this compound on growing plants, 

 nor upon the variations in that action which exhibit themselves 

 in different soils and in the case of different plants. In the 

 service both of physiology and of agriculture, it is to be wished 



