300 Royal Institution .— 



are sweet, others are bitter; some have delightful perfumes, others 

 have dreadful smells ; some are wholesome food, others the most 

 powerful poisons known to man. 



In spite of this wonderful diversity in their properties, all the 

 specimens on this tray are compounds of carbon with a very few 

 elements. Carbon, hydrogen, oxygen, and nitrogen are the only 

 elements which occur in this collection of substances. Some of 

 these substances contain carbon and hydrogen ; some contain carbon, 

 hydrogen, and oxygen; some, carbon, hydrogen, and nitrogen; and 

 some again contain carbon, hydrogen, oxygen, and nitrogen. But 

 not one of the specimens on this tray contains anything besides these 

 four elements. 



There is no difficulty in resolving any one of these substances into 

 its ultimate elements. This sugar, for example, on being heated to 

 redness in a tube, leaves a black deposit which is carbon, whilst a 

 liquid which is water distils over. If we were to electrolyze this 

 liquid, we should obtain hydrogen and oxygen, and so we should 

 exhibit carbon, hydrogen, and oxygen obtained from sugar. Again, 

 instead of heating this sugar in the tube without allowing the air free 

 access to it, we might burn it in excess of oxygen. If we were to do 

 so, we should obtain carbonic acid and water ; and moreover all the 

 carbon in the sugar would assume the form of carbonic acid, and all 

 the hydrogen the form of water. So we can obtain carbon and 

 hydrogen either in the free state or in the very common and well- 

 known forms of combination as carbonic acid and water. Nitrogen, 

 when it is present, can be made to assume the form of free nitrogen. 

 For that purpose, all that is requisite is to heat the substance to red- 

 ness with excess of oxygen, and to adopt certain precautions to avoid 

 the production of oxide of nitrogen. 



Thus the pulling to pieces of these substances on the tray is a 

 matter of very little difficulty ; more than fifty years ago chemists 

 could do that; but how to put the pieces together again is a much 

 more difficult task. 



Sugar consists of 72 parts by weight of carbon, 11 parts of hy- 

 drogen, and SS parts of oxygen. We may bring together carbon, 

 hydrogen, and oxygen in these proportions, and shake them up to- 

 gether, or heat them or cool them, and yet we shall never get them to 

 combine so as to form sugar. Alcohol consists of 24 parts of carbon, 

 G parts of hydrogen, and 1G parts of oxygen, but no alcohol ever 

 results from making such a mixture. Neither sugar nor alcohol can 

 exist at the temperature to which it is requisite to raise our mixture 

 of carbon, hydrogen, and oxygen in order to get chemical action to 

 set in. At ordinary temperature the organic elements will not enter 

 into combination ; whilst at high temperatures they combine, it is 

 true, but yield comparatively very few compounds. 



It was long after chemists had effected the analysis of organic 

 bodies before they learnt how to effect the synthesis of even one of 

 them ; and hence the belief sprung up that organic products, such as 

 those on our tray, were intrinsically different from mineral products. 

 Whilst stones, water, and the like were regarded as having their 



