176 - IMPORTANCE OF THE SUBJECT. 



feebleness of effect which takes place by reason of the dispersion of the incident beam 

 through the action of the prism, and the great loss of light through reflection from its 

 surface, it might appear a difficult operation to effect a determination in this way. En- 

 couraged, however, by the purity of the skies in America, I made the trial, and have met 

 with complete success. 



776. Before entering on the experiments which I have to communicate, I cannot 

 avoid once more impressively calling the attention of chemists to the true character of 

 those emanations which are here designated "tithonic rays." It is riot enough that we 

 admit the existence, throughout the spectrum, of dark rays, possessing the power of 

 bringing about chemical changes ; it is not enough that we call them chemical rays ; 

 there are qualities of distinction appertaining to them which mark them out as being 

 specific in their kind, properties which they possess totally distinct from those of light 

 and heat. Their title to the rank of a distinct imponderable agent is just as perfect as 

 that of light or heat. From heat they are to be distinguished by incapacity for metallic 

 conduction, and by want of the power of expanding bodies; from light, by failing to 

 give any impression to the organ of vision. According to the recognised rules of chem- 

 istry, they ought to be received as a fourth imponderable agent. 



777. It is not sufficient, as has been said, to call them " chemical rays." The term 

 implies that the distinctive characteristic pertaining to them is the power of changing 

 the composition of bodies. But do not the rays of heat eminently produce like chan- 

 ges I Are not half the decompositions in chemistry brought about by the action of ca- 

 loric ? As respects LIGHT, many instances are already known in which it produces de- 

 compositions and combinations ; as will be presently shown, it is the agent that brings 

 about the decomposition of carbonic acid. The faculty of producing a like effect is not 

 the distinguishing quality of the tithonic rays, nor can the term chemical be any more 

 applied to them than to either of their acknowledged distinct companions. Unless, 

 therefore, chemists are content to admit that a species of heat may exist devoid of the 

 yower of expanding bodies, of giving the sensation of warmth, and of being trans- 

 mitted by conducting processes ; or, unless they admit that light can exist in such a 

 modified condition as to produce in our eyes the sensation of darkness, they will have to 

 admit these tithonic rays as constituting a fourth imponderable agent. The name they 

 may take is not a matter of importance; that which is least trammelled by hypoth- 

 esis is best. It is not the object of this and the foregoing chapters to show merely 

 that a class of invisible rays exists in the spectrum ; that has been known for a long 

 time ; but it is to point out the true relation of these rays to other bodies and other 

 forces in the world, to assert for them their title of a fourth distinct imponderable agent, 

 and to secure for them the admission of that title by giving them a name. 



778. When the leaves of plants are placed in water from which all air has been ex- 

 pelled by boiling, and exposed to the sun's rays, no gas whatever is evolved from them. 

 When they are placed in common spring or pump water, bubbles quickly form, which, 

 when collected and analyzed, prove to be a mixture of oxygen and nitrogen gases ; 

 from a given quantity of water a fixed quantity of air is produced. When they are 

 exposed in water which has been boiled and then impregnated with carbonic acid, the 

 decomposition goes on with rapidity, and large quantities of gas are evolved. 



