28 NATURE AND OBJECTS OF PHYSIOLOGICAL SCIENCE. 



doubt whatever, that this directing power is only another manifestation 

 of the same magnetic attractiveness; depending on the relation between 

 the magnetized bar, or needle, and the Earth, which must itself be re- 

 garded as a great magnet. Hence the idea of a peculiar kind of mutual 

 attractiveness, existing in only a limited class of bodies, capable of being 

 excited in one by a certain agency on the part of the other,* and re- 

 quiring for its exercise or manifestation a certain set of conditions, with- 

 out which no phenomenon results, is that which we regard as funda- 

 mental in the Science of Magnetism. 



22. We may now turn from these departments of Physical Science to 

 Chemistry ; and here we shall find that the investigation is carried on 

 upon the very same plan. In fact, the whole science of Chemistry is 

 founded upon the idea of a certain attractiveness or affinity existing 

 among the ultimate particles or molecules of the different elementary 

 substances ; and therefore entirely distinct from the homogeneous attrac- 

 tion, which holds together the particles of the same mass, or from the 

 gravitative attraction, which operates alike upon all masses, whatever be 

 their composition. Thus we say that Sulphuric acid and Potash have an 

 affinity for each other ; because they unite when they are brought 

 together, and form a new compound. This is a Ctoemical action or phe- 

 nomenon. Now we know that they tend to unite with a certain force ; 

 a force, however, which cannot be measured mechanically, and which 

 can only be expressed by comparing it with some other force of the 

 same kind. Thus we say that the mutual affinity of Sulphuric acid and 

 Potash is greater than that of Nitric acid and Potash ; because, if we 

 pour Sulphuric acid upon Nitrate of Potash, the Sulphuric acid detaches 

 (as it were) the Potash from its connexion with the Nitric acid, forms a 

 new compound with it, and sets the Nitric acid free. Hence we say 

 that it is a property of Sulphuric acid to have a very strong affinity for 

 Potash. This property exists in every particle of Sulphuric acid that 

 exists, whether free or combined ; but it does not manifest itself, except 

 when called into operation by the contact of Potash ; and it then de- 

 velopes a force, which may completely change the combinations previ- 

 ously existing, and give rise to new ones. 



23. Now of this property we are not informed by any of the other 

 properties of Sulphuric acid ; and we only recognise its existence by the 

 action which is the result of its exercise. If a new element or com- 

 pound be discovered, the chemist is totally unable to predict its force of 

 affinity for this or that substance ; and he can only guess by analogy, 

 what will be its behaviour under various circumstances. Thus if it have 

 the external properties of a Metal, he presumes that it will correspond 

 with the Metals in possessing a strong affinity for oxygen, sulphur, &c.; 

 whilst if it seem analogous to Iodine, Chlorine, &c., he infers that it will 

 be a supporter of combustion, that it will form an acid with hydrogen, 

 and so on. But, even though such guesses may be made with a certain 

 amount of probability, nothing but experience can show the positive 

 degrees of affinity which the substance may have for others of different 

 kinds ; and the experimental determination can only be made, by observ- 



* As "when one magnet is made by another; or when iron rails, pokers, &c., become 

 magnetic by the influence of the earth. 



