X

INTRODUCTION.

effects of various substances on plants; his chemicalspeculations, however, are for the most part of littlevalue, though he was often apparently on the point ofmaking important discoveries. The discovery of carbonic-acid-gas or fixed air, by Dr. Black, and the beautifulexperiments of Priestley, opened a new field of inquiry |and research; he observed that plants possessed theproperty of purifying the air, in fact that they wereable to decompose the carbonic-acid-gas which it alwayscontains in small quantity ; appropriating the carbon,and restoring back to the atmosphere the oxygen, orvital air, so necessary to the processes of respiration andcombustion. The knowledge of this great fact, neces-sarily led to many minor discoveries respecting the ;growth of plants, and the sources of their food. Afterthis period, Organic Chemistry began to attract a largeshare of the attention of chemists, the composition ofvegetable substances was carefully investigated, new 1modes of analysis were discovered, and an immense |mass of curious and useful facts was collected. A great |number of chemists occupied themselves with researches j

in Vegetable Chemistry, but for the most part they were i

i

employed in examining the innumerable substances i'which plants produce ; whilst the great questions as tothe food of plants, their growth, and nourishment, were i