INTKoDHTION 13 



previous observers (the transformations of the metals into their earths 

 or oxides), had been explained. 



4. In order to illustrate by experiment one more example of 

 chemical change and the application of the law of the indestructi- 

 bility of matter, we will take some common table salt and lunar 

 caustic, which is well known from its use in cauterising wounds. By 

 taking a clear solution of each and mixing them together, it will at 

 once be remarked that a solid white substance is formed, which settles 

 to the bottom of the vessel, and is insoluble in water. This substance 

 may be separated from the solution by filtering ; it is then found to be 

 an entirely different substance from either of those taken originally 

 in the solutions. This is evident from the fact that it does not 

 dissolve in water. On evaporating the liquid which passed through 

 the filter, it will be found to contain a new substance unlike either 

 table salt or lunar caustic, but, like them, soluble in water. Thus 

 table salt and lunar caustic, two substances soluble in water, being 

 taken, by their mutual chemical action produced two new substances, 

 one insoluble in water, and the other remaining in solution. Here, 

 from two substances two others are obtained, consequently there 

 occurred a reaction of substitution. The water served only to convert 

 the acting substances into a liquid and mobile state. If the lunar caustic 

 and salt be dried 21 and weighed, and if about 58.^ parts by weight for 

 instance, grams 2 ' 2 of salt and 170 grams of lunar caustic be taken, 

 then 143^ grams of insoluble silver chloride and 85 grams of sodium 

 nitrate will be obtained. The sum of the weights of the acting and 

 resultant substances are seen to be similar and equal to 228^ grams, 

 as necessarily follows from the law of the indestructibility of 

 matter. 



21 Drying is necessary in order to remove any water which may be held in the salts 

 (see Note 17). If the original and resultant substances be dried, then the water 

 employed for solution, and which is removed in drying, may be taken in indefinite 

 quantities. 



The exact weights of the acting and resulting substances are determined with the 

 greatest difficulty, not only from the possible inexactitude of the balance (every weighing 

 is only correct within the limits of the sensitiveness of the balance) and weights used 

 in weighing, not only from the difficulty in making corrections for the weight of air dis- 

 placed by the vessels holding the substances weighed and by the weights themselves, 

 but also from the hygroscopic nature of many substances (and vessels) causing absorption 

 of moisture from the atmosphere, and from the difficulty in not losing any of the substance 

 to be weighed in the many operations (filtering, evaporating, and drying, &c.) which have to 

 be gone through before arriving at a final result. All these circumstances have to be 

 taken into consideration in exact ivs.-uivlu's, and their elimination requires very many 

 special precautions which are impracticable in preliminary experiments ; these arrive- 

 within only a certain comparatively rough proximity to those weights (expressed by 

 chemical formulae) which (all with a certain, definite, and inevitable error) correspond 

 with reality. 



