Volume 1

The principles of chemistry / by D. Mendeléeff.

  • Dmitri Mendeleev
Date:
1905
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: The principles of chemistry / by D. Mendeléeff. Source: Wellcome Collection.

    36/674 (page 6)
    о The majority of the chemical changes which occur in nature and are made use of technically are very complicated, as they consist of an association of many separate and simultaneous combinations, decom- positions, and replacements. It is chiefly due to this natural complexity of chemical phenomena that 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,11 yet their real nature was unknown, and they could not be predicted 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 forms 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 understanding the invisible, but ponderable, gaseous and vaporous forms of matter, it was impossible to obtain any fundamental chemical knowledge, because the gases formed or used up in any reaction escaped notice. 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 observed. When wood burns, it undergoes a chemical change into gaseous products, which escape as smoke. The substance of the wood is not lost; it is only converted into gaseous form by a chemical process. 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 either com- bustion 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 wcod combine with a portion of the air; in like manner iron increases in weight by rusting. W hen gunpowder is burnt its substance is not destroyed, but only converted into gases and smoke. So also in the growth of a tree : the seed does not increase in mass of itself and from itself, but grows because it absorbs gases from 11 For example, the conversion of the juice of grapes, containing a saccharine principle (glucose), into wine or vinegar, and the extraction of metals from the ores found in the earth’s crust.