36 



SCIENCE. 



In another place right across the whole face of the 

 lake stretched half a dozen islands, affording no foothold 

 for man or beast, surrounded by stagnant green water 

 filled with every conceivable vegetable rottenness. 



The sewers from farm houses, cottages of laborers 

 and factories were noticed to draindirectly in the water 

 supply ; in fact the source of water supply of New 

 York city was found to be a common drain for 

 about 300 cattle yards, dwelling-houses, factories, 

 pig-sties, slaughter-houses, and other sources of im- 

 purities, every one of which are distinctly shown on the 

 maps we present, the exact location being indicated by a 

 black spot. Space will not allow us to give futher evi- 

 dence on this point which it is in our power to offer, but 

 we present a cut of one of these sources of pollution, show- 

 ing the direct drainage into the Croton water. 



Of the danger of drinking such water full of the vilest 

 contaminations we will not dwell, each reader can take 

 his own course, but those who are prudent will both boil 

 and filter it before using for drinking purposes. Indi- 

 viduals and journals still claim that the source of the 

 supply is free from contaminations, and the water pure 

 and fit for drinking purposes ; to be consistent they have 

 to say that the water is wholesome. 



Professor Lcds of the Stevens Institute recently showed 

 that the Croton water contained more organic volatile 

 mattert han the water supply of Newark, which is taken di- 

 rect from the Passaic with all the sewage of Paterson and 

 other towns. He found the organic matter in 100, coo 

 parts in New York water to be 6,50, Newark 6.00, Hobo- 

 ken 4.50. At the February meeting of the American 

 Chemical Society (see page of this number), Dr. E. 

 Waller, of the New York Board of Health, endeavored 

 to deny this startling statement, by producing analyses 

 of his own, showing quite different results. We under- 

 stand that at the March meeting of the same Society, 

 Professor Leeds asserted to the satisfaction of the Society 

 that Dr. Waller's methods were bad and had led him to 

 error, while the integrity of his own analysis was estab- 

 lished. 



We consider the method ol storing the water supply 

 of a city in shallow, marshy lakes, in fever and malarious 

 districts, to be wrong in principle, and that a radical 

 change in the management of the water supply of New 

 York City, rather than an expensive extension of it, to be 

 the most prudent course to adopt at the present moment. 



ELECTRIC CONDUCTION AND DISCHARGE. 



By F. E. Upton. 



The question of the nature and the vehicle of the elec- 

 trical discharge is an important one, and its determina- 

 tion will contribute greatly to the solution of many inter- 

 esting problems in cosmical physics. It is desired in 

 this article to call attention to some recent advances that 

 have been made in this direction. 



The view that the phenomenon is one of pure conduc- 

 tion, though it has received the attention of eminent phy- 

 sicists, can be said to be no longer entertained. 



When a conductor is made to connect two poles or 

 electrodes which are at a different potential, it is well 

 known that the greater the cross section of the conduc- 

 tor, or in other words, the more of the conducting ma- 

 terial is laid bare by a cross section, the less resist- 

 ance will be offered to the union of the electricities of 

 the two terminals, and the greater will be the ensuing cur- 

 rent, with a given E. M. F. 



Now, in the discharge, the contrary is observed 

 directly. This characterist : c of conduction is absent when 

 the discharge takes place; in a tube containing air, the 

 greater the pressure (above a certain inferior limit), or the 

 more of the conducting material there is laid bare by a cross 

 section, the greater will be the resistance to the passage of 

 the spark, and the nearer together the terminals will have to 



be brought to effect a spark with a given difference of 

 potential. Sir Wm. Snow Harris, in 1834, made an at- 

 tempt to grasp at the law governing the relation of the 

 length of spark to pressure ; and he then stated that the 

 length of spark is in the simple inverse ratio of the pres- 

 sure. Gordon, in 1878, made a series of experiments to 

 test this law, (Elec. and Mag. II. 55-62). He found that 

 from a pressure of about eleven inches to that of the at- 

 mosphere, Harris's law held approximately good. 



Representing resistance by r, and matter laid bare by 



cross section by s, in the case of conductor r = — ; in 



s 



the case of discharge 92 of r=s, approximately. Thus there 

 is in question two entirely different order of phenomena. 



Another distinctive characteristic of conduction will be 

 recognized in the fact that whenever there is any conduc- 

 tor at all, however small and however long it may be, 

 connecting two poles, some degree of current will flow, 

 as long as there is any difference of potential. With dis- 

 charge, however, a certain lower limit of distance apart 

 of poles, and of interposed matter, is requisite for any 

 current at all, and when that limit is reached the spark 

 passes, instantaneously, and the discharge commences. 



Whether the current passes by conduction or dis- 

 charge, heat is equally developed ; in the conductor in 

 the one case, and in the interposed matter in the other. 

 This common development of heat does not in any way 

 assimilate the two phenomena. The condition of affairs 

 in the two cases will perhaps become obvious if recourse 

 is had to the corresponding hydraulic analogy. 



Imagine a pond of water held in place by a dam, with 

 a pipe leading from the bottom of the dam, for the pur- 

 pose of drawing water from the pond. The smaller that 

 pipe is in section, the smaller will be the current of water 

 flowing through it under a given head, and a certain 

 amount of heat will be developed by friction of the water 

 against the interior of the pipe ; moreover some degree 

 of current will flow as long as the pipe has any size of 

 cross section at all. That corresponds to conduction. 

 Now let the pipe be imagined closed to the exit of water ; 

 as long as the dam is sufficient, no current at all will 

 flow ; but suppose the dam be diminished in thickness 

 gradually and constantly, a point will be eventually 

 reached when it will no longer suffice to hold back the 

 water, which will break through the impediment ; the 

 friction of the water against the fragments of the dam, 

 and of those fragments against each other will develop 

 heat as in the first case. That corresponds to the dis- 

 charge. 



By this analogy the difference between conduction and 

 discharge is clearly apparent. A conductor between 

 two points at a different potential never offers any resis- 

 tance to the passage of the current, strictly speaking. 

 Instead of saying that a slender wire offers more resist- 

 ance than a thick one, it would give a better understand- 

 ing of the matter to say of the latter that it offered a freer 

 passage to the current than the former. In the case of 

 discharge, on the contrary, the matter interposed between 

 the points acts always as a liar, or resistance to be over- 

 come, and the more there is of it the more resistance. It 

 is never an aid or way. 



Mr. E. Goldstein, in the Annalen der Physik, de- 

 scribes an ingenious experiment bearing upon this point, 

 which, if not conclusive, is entitled to some considera- 

 tion. In a discharge tube which was filled with dry ni- 

 trogen, he placed a little sodium, which could be vapor- 

 ized by heating. The positive light had a purplish red 

 color, but in the vicinity of the sodium it was of a golden 

 yellow. By careful heating and manipulation, the upper 

 half of the tube could be kept red and the lower halt yel- 

 low. Now the tube was brought over and near, in a 

 horizontal and equatorial position, to a powerful magnet. 

 The discharge light was repelled as a slender thread to 

 the opposite (upper) side of the tube ; but it was a pure 

 eddish thread, and showed no trace of sodium yellow. 



