ON THE GENERAL DIFFUSION OF KNOWLEDGE. 



corrosion. These effects are now ex 

 plained on the principles of galvanism. When 

 two metallic substances of different kinds are 

 connected by moisture, they form what is called 

 a galvanic circle ; and, therefore, when one kind 

 of metal is placed in contact with another, if 

 either water or the moisture of the atmosphere 

 adheres to them, a galvanic circle is formed, 

 and oxidation is produced. On this ground the 

 late Sir Humphrey Davy suggested the propriety 

 of fastening the upper sheathing of ships with 

 copper instead of iron nails. The same princi 

 ple may be rendered of extensive application, 

 and may afford many useful hints to every arti- 

 zan employed in working and combining metals. 

 A knowledge of magnetism might also, in 

 many cases, be directed to useful practical 

 applications. This mysterious power, in con 

 nexion with its polarity, has already enabled the 

 miner and surveyor to traverse the remotest 

 corners of the largest mines, and to trace their 

 way back in safety through all the windings of 

 those subterraneous apartments, and has direct 

 ed the navigator to steer his course with cer 

 tainty, through the pathless ocean, to his &quot; de 

 sired haven.&quot; Throughout all the regions of 

 the globe the magnetic power extends its influ 

 ence ; and it is now found to have an intimate 

 connexion with heat, electricity, and galvanism. 

 Of late years, it has been ascertained that iron 

 with its oxides and alloys are not the only sub 

 stances susceptible of magnetic influence. The 

 magnetism of nickel, though inferior to that of 

 iron, is found to be considerable ; and that of 

 cobalt and titanium is quite perceptible. Nay, 

 the recent discoveries of Arago have shown, 

 &quot; that there is no substance but which, under 

 proper circumstances, is capable of exhibiting 

 unequivocal signs of the magnetic virtue.&quot; In 

 consequence of a recent discovery of M. Oer 

 sted, &quot; we are now enabled to communicate, at 

 and during pleasure, to a coiled wire, of any 

 metal indifferently, all the properties of a mag 

 net its attraction, repulsion, and polarity, and 

 that even in a more intense degree than was 

 previously thought to be possible in the best 

 natural magnets.&quot; This discovery tends to en 

 large our views of the range of magnetic influ 

 ence, and to lead us to the conclusion that its 

 powers may hereafter be applied to purposes of 

 which at present we can have no conception. 

 Although the polarity of the magnet has been 

 of incalculable service to mankind, particularly 

 in promoting navigation and enlarging our 

 knowledge of the globe, yet we have no reason 

 to believe that this is the only practical purpose 

 to which its powers may be applied, or the only 

 reason why the Creator has so widely diffused 

 its influence in the system of nature ; since, in 

 his diversified operations in the material world, 

 he so frequently produces a variety of effects 

 from one and the same cause. It remains with 



man to prosecute his observations still more ex 

 tensively on this subject, and his induscry wiK, 

 doubtless, be rewarded wfth the discovery of 

 new relations, laws, and combinations, which 

 may be susceptible of the most important practi 

 cal applications in the arts which minister to 

 the comfort and convenience of mankind. Even 

 in its present state, the attractive property of 

 magnetism is capable of being applied as a me 

 chanical power, in certain pieces of machinery, 

 although its application in this way has never 

 yet been attempted on an extensive scale. 



The following fact shows how its attractive 

 power has lately been applied to the prolonga 

 tion of life, and the warding off of incurable 

 disease, in the case of a useful class of our 

 fellow men. &quot; In needle manufactories the work 

 men are constantly exposed to excessively mi 

 nute particles of steel which fly from the grind 

 stones, and mix, though imperceptible to the 

 eye, as the finest dust in the air, and are inhaled 

 with their breath. The effect, though imper 

 ceptible, on a short exposure, yet being constantly 

 repeated from day to day, produces a consti 

 tutional irritation, dependent on the tonic pro 

 perties of the steel, which is sure to terminate in 

 pulmonary consumption ; insomuch, that persons 

 employed in this kind of work used scarcely ever 

 to attain the age of forty years. In vain was it 

 attempted to purify the air, before its entry into 

 the lungs, by gauzes or linen guards ; the dust 

 was too fine and penetrating to be obstructed 

 by such coarse expedients, till some ingenious 

 person bethought him of that wonderful power, 

 which every child that searches for its mother s 

 needle with a magnet, sees in exercise. Masks 

 of magnetized steel wire are now constructed 

 and adapted to the faces of the workmen. By 

 these the air is not merely strained but searched 

 in its passage through them, and each obnox 

 ious atom arrested and removed.&quot;* 



This interesting fact affords a striking proof 

 of the useful purposes to which the powers and 

 properties of natural substances maybe applied, 

 when the mind is directed to contemplate them 

 in all their bearings, and to trace them to all 

 their legitimate consequences. The attractive 

 power of the magnet, considered not only in its 

 relation to iron and steel, but to all other sub 

 stances in which magnet ical virtue is found in a 

 greater or less degree to reside might, there 

 fore, in the hands of an ingenious mechanic, lead 

 to many interesting experiments, which might 

 pave the way for the most important practical 

 results. 



The facts connected with the science of Geo 

 logy may likewise, in many instances, be direct 

 ed to practical purposes. From the researches 

 which, of late years, have been made in the in 

 terior of the earth, geologists are now pretty well 



Herschel s Prclrn. Dis. on Na/. Philos. 



