326 Royal Institution, 



greatei* pressure of air necessary to produce the greater intensity, 

 would at the same time increase the number of pulsations, and so 

 raise the tone ; but to prevent this, the glottis must at the same 

 time be lengthened, and vice versa ; or, in other words, that the dif- 

 ferent lengths of the glottis can, under diiFerent degrees of pressure, 

 produce the same number of shocks, but at diiferent degrees of 

 intensity. 



Of the Qualities of the Voice. 

 Various simultaneous causes modify the qualities of the voice : — 

 1, according as the glottis partially or entirely closes the passage 

 between the explosions, it produces veiled or brilliant sounds ; 2, the 

 tube which surmounts and surrounds it also greatly affects the 

 quality of the voice ; by its contractions it gives brilliancy to it and 

 its widening volume ; 3, the ejjiglottis also plays a very important 

 part, for every time that it lowers itself, and nearly closes the orifice 

 of the larynx, the voice gains in brilliancy ; and when, on the other 

 hand, it is drawn up, the voice immediately becomes veiled. 



ROYAL INSTITUTION OF GREAT BRITAIN. 



June 1, 1855. — "On the Currents of the Leyden Battery." By 

 Professor Tyndall, F.R.S. 



In our conceptions and reasonings regarding the forces of nature, 

 we perpetually make use of symbols, which, when they possess a 

 high representative value, we dignify with the name of theories. 

 We observe, for example, heat propagating itself through a bar of 

 metal, and help ourselves to a conception of the process by com- 

 paring it with water percolating through sand, or travelling by 

 capillary attraction through a lump of sugar. In some such way 

 we arrive at what is called the material theory of heat. The thing 

 seen is thus applied to the interpretation of the thing unseen, and 

 the longing of the human mind to rest upon a satisfactory reason, is 

 in some measure satisfied. So also as regards the subject of the 

 present evening's discourse ; we are not content with the mere facts 

 of electricity ; we wish to look behind the fact, and, prompted by 

 certain analogies, we ascribe electrical pha?nomena to the action of a 

 peculiar fluid. Such conceptions have their advantages and their 

 disadvantages : they aff'ord peaceful lodging to the intellect for a 

 time, but they also circumscribe it; and by and by, when the mind 

 has grown too large for its mansion, it often finds a difficulty in 

 breaking down the walls of what has become its prison instead of 

 its home. Thus, at the present day, the man who would cross the 

 bounds which at present limit our knowledge of electricity and 

 magnetism finds it a work of extreme difficulty to regard facts in 

 their simplicity, or to rid them of those hypothetical adornments 

 with which common consent has long invested them. 



But though such is the experience of the earnest student of 

 Natural Philosophy at the ]5resent — though he may be compelled to 

 refuse his assent to the prevalent theoretic notions, he may never- 

 theless advantageously make use of the language of these theories in 

 bringing the facts of a science before a jjublic audience ; and in 

 speaking of electricity, the speaker availed himself of the convenient 



