INTRODUCTION 9 



animals and plants. So closely is it bound up with the vita! 

 phenomena that wherever we find life we find protoplasm, and 

 vice versa wherever that substance is found we find manifestations 

 of vitality. This being the case, we sometimes say the protoplasm 

 is the physical basis of life, or the same idea is expressed by saying 

 that life is a property of protoplasm. From the universality of its 

 occurrence in organisms, and the fact that it is indissolubly bound 

 up with vital activities, it is obvious that protoplasm is a substance 

 of extreme importance. 



It is a very difficult material to deal with chemically, because 

 when we place it in a test tube, or submit it to the ordinary methods 

 of chemical analysis, we at once kill it, and so are no longer dealing 

 with living matter. Ultimate analysis shows that there is no 

 chemical element to be found in protoplasm that we cannot also 

 find in the inorganic world, so that we cannot regard living and 

 non-living as two essentially different sorts of matter, but only 

 as the same matter in different stages. 



While this is true of the elements contained in the living sub- 

 stance, it does not hold for the compounds. Chemical analysis 

 shows us that protoplasm is an intimate mixture of a number of 

 different classes of compounds mainly falling under five headings, 

 viz. : water, inorganic salts, fats, carbohydrates and proteins. 

 Of these the first two are also met with in inorganic matter, but 

 the last three are only found as a part of, or as the products of, 

 protoplasm, and encountered nowhere else in nature. They are 

 for this reason spoken of as organic compounds. Fats and Carbo- 

 hydrates (e.g. starches and sugars) are composed of the elements 

 Carbon, Hydrogen and Oxygen in various proportions. Proteins, 

 the most complicated compounds that we know, always contain 

 in addition to these elements Nitrogen, and usually several others, 

 such as Sulphur, Phosphorus, Sodium, Magnesium, Iron, etc. 



In addition to this we can ascertain by experiment that when a 

 living being dies there is no loss of weight ; a very important point, 

 since it demonstrates that the cessation of life is not accompanied 

 by the loss of any material substance. There is, however, a remark- 

 able change in both its chemical and physical properties, and we 

 must assume that this is due to a rearrangement of the composition 

 and minute physical structure of the protoplasm, as it is not due 

 to the withdrawal of anything tangible. 



Physics teaches us that energy, which is necessary for the 

 performance of any work, exists in two forms. Firstly, we have 

 kinetic energy, the variety of energy that expresses itself in the 

 form of motion, heat, light and electricity, and is brought promi- 

 nently to our notice by its results. Secondly, potential energy is 



