1853] WRITINGS OF JOSEPH HENRY. 311 



bodies and renders it the obedient slave of his will — the 

 patient drudge, which in a thousand ways administers to his 

 wants, his convenience, and his luxuries, and enables him 

 to reserve his own energy for the higher purpose of the de- 

 velopment of his mind and the expression of his thoughts. 

 The following is a list of all the primary powers which as 

 yet have been used by man in accomplishing his varied pur- 

 poses in the wide domain of practical life. These are : 



1. Water power, 



2. Wind power, 



3. Tide power, 



4. The power of combustion, and 



5. The power of vital action. 



To this list may hereafter be added the power of the vol- 

 cano and the internal heat of the earth ; and besides these 

 science at the present time gives no indications of any other. 

 These are denominated primary powers, though in reality, 

 when critically studied, they may all, except the two last 

 mentioned, be referred to actions from without the earth, and 

 principally to emanations from the sun. 



Gravitation, electricity, galvanism, magnetism, and chem- 

 ical affinity can never be employed as original sources of 

 power. At the surface of the earth they are forces of quies- 

 cence, the normal condition of which must be disturbed be- 

 fore they can manifest power, and then the work which they 

 are capable of performing is only the equivalent of the power 

 which was communicated to them. 



There is no more prevalent and mischievous error than 

 the idea that there is in what are called the " imponderables " 

 a principle of spontaneous activity. Heat is the product of 

 chemical action, and electricit}"^ only manifests power when 

 its equilibrium is disturbed by an extraneous force, and then 

 the effect is only proportional to the disturbing cause. It 

 was for this reason that the existence of electricity remained 

 so long unknown to man. Though electricity is not in itself 

 a source of power, yet from its extreme mobility and high 

 elasticity it affords the means of transmitting power with 



