FOUM AND SIZE OF PARTICLES EMPLOYED IN CONSTRUCTION OF PLANTS. 571 



molecules of uiiiorphous calciuiii uud uinorphous silica; but this much must be 

 said with regard to these depositions, that they cannot be looked upon as organized 

 substances. 



Here is the proper place to consider investigations as to the size of molecules. 

 In these researches, especially for tlie aseurtiiiniiieiit of the size of molecules of 

 gas, very various physical facts offer themselves as data, such as the coefficients 

 of condensation, the deviations from Boyle's law, the variability of the coefficients 

 of expansion, the heat of evaporation, and, finally, the constants of dielectrics. 

 The results ditier considerably. For example, the estimates of sizes given for a 

 certain gas by difTerent methods differ from one another far more than those 

 which have been obtained from different gases by one and the same method. But 

 all calculations agree that the diameter of the hypothetically spherical molecules 

 of gas must lie between the hundred-thousandth and the millionth part of a 

 millimetre, and that these limits cannot be overstepped, either above or below, to 

 any great extent even in the exti-emest cases. A cubic millimetre of gas would 

 therefore contain about 866 billions of molecules, and if the gas were condensed 

 into a fluid, the number in a millimetre would increase to a trillion. 



The lenrth of lisht-waves is of the smallest of measurable dimensions. If 

 the diameter of a molecule is taken in round numbers at the millionth part of a 

 millimetre, this is 700 times smaller than the wave-length of red light, and the 

 diameter of a molecule bears about the same proportion to a millimetre, as a 

 millimetre to a stretch of 2 kilometres. Particles of these dimensions are 

 bej'ond the conception of our senses; even the highest powers of the microscope 

 are unable to disclose them to us, as is shown by the following considerations. 

 Sheets of gold-leaf are produced, whose thickness amounts to only a hundredth 

 part of the wave-length of light, and which accordingly contain only 3-5 molecules 

 of gold above one another. These gold-leaves are transparent to white light, 

 and this may be regarded as a proof that rays of light penetrate through the 

 chinks between the molecules. Nevertheless this leaf appears as a continuous 

 mass under the best microscopes, and it is not possible to recognize the individual 

 molecules composing it. Under the most favourable circumstances, our microscopes 

 are able to render visible only particles which comprise perhaps two million 

 molecules. Since there are no certain data to enable us to measure how great 

 is the number of molecules from which raicelke are built up, and in what manner 

 the molecules are grouped in them, it would be rash to attempt any conjectures 

 as to the size of micellae. The possibility of perceiving micellae with the micro- 

 scope in their outline and shape, especially those of albuminous bodies, whose 

 molecules are composed of such a large number of atoms (see p. 45G) is not to be 

 wholly excluded, particularly since our microscopes are still capable of much 

 improvement. Still, the probability is but a remote one, and as matters stand at 

 present, all conclusions on this subject would be of the nature of theory, in 

 which one uncertain hypothesis has to furnish the foundation for a second, 

 still more doubtful. 



