INTRODUCTION 5 



separated out, and green vitriol (iron sulphate, which only differs from 

 the blue vitriol in that the iron has replaced the copper) is obtained in 

 solution. In this manner iron may be coated with copper-, so also copper 

 with silver ; such reactions are frequently made use of in practice. 



The majority of the chemical changes accomplished in nature and 

 the arts are very complicated, as they consist of an association of many 

 separate and simultaneous combinations, decompositions, and replace- 

 ments. In this natural complexity of chemical phenomena is discovered 

 the chief reason why for so many centuries chemistry did not exist as 

 an exact science ; that is to say, that although many chemical changes 

 were known and made use of, 10 yet their real nature was unknown, nor 

 could they be foreseen or directed at will. Another reason for the 

 tardy progress of chemical knowledge is the participation of gaseous 

 substances, especially air, in many reactions. The true comprehension 

 of air as a ponderable substance, and of gases in general as peculiar elastic 

 and dispersive states of matter, was only arrived at in the sixteenth and 

 seventeenth centuries, and it was only after this that the transformations 

 of substances could form a science. Up to that time, without under- 

 standing the invisible and yet ponderable gaseous and vaporous states 

 of substances, it was impossible to form any fundamental chemical 

 evidence, because gases escaped from notice between the acting and 

 resultant substances. It is easy from the impression conveyed to us by 

 the phenomena we observe to form the opinion that matter is created 

 and destroyed : a whole mass of trees burn, and there only remains a 

 little charcoal and ash, whilst from one small seed there grows little 

 by little a majestic tree. In one case matter seems to be destroyed, and 

 in the other to be created. This conclusion is arrived at because the 

 formation or consumption of gases, being under the circumstances 

 invisible to the eye, is not noted. When wood burns it undergoes a 

 chemical change into gaseous products, which escape as smoke. A very 

 simple experiment will prove this. By collecting the smoke it may be 

 observed that it contains gases which differ entirely from air, being 

 incapable of supporting combustion or respiration. These gases may 

 be weighed, and it will then be seen that their weight exceeds that of 

 the wood taken. This increase in weight arises from the fact that, in 

 burning, the component parts of the wood combine with a portion of 

 the air ; in like manner iron increases in weight by rusting. In burn- 

 ing gunpowder its substance is not destroyed, but only converted into 

 gases and smoke. So also in the growth of a tree ; the seed does not 



10 Thus tin- ancients knew how toconvert the juice of grapes containing the saccharine 

 principle (glucose) into wine or vinegar, or how to extract metals from the ores which 

 are found in the earth's crust, and how to prepare glass from earthy substances. 



