232 



NATURE 



[July 5, 1888 



Prof. Nichols electroplates a strip 1 millimetre wide of the car- 

 bon rod parallel to the axis with copper to the required thick- 

 ness. The influence of temperature up to 100 C. is then entirely 

 imperceptible. 



Messrs. Glazebrook and Fitzpatrick have once more 

 utilized the resources of the Cavendish Laboratory to determine 

 the specific resistance of mercury, and therefore the value of 

 the ohm (io 9 C.G.S. units of resistance). The result, together 

 with the most recent determinations, is given in the following 

 table :— 



Value of Value of 

 Siemens ohm in 

 Observer. Date. unit in centimetres 



B.A. of mercury 

 units. at 0°. 



Lord Rayleigh and Mrs. Sidgwick ... 1883 095412 106*23 



Mascart, Nerville, and Benoit 1884 0-95374 106-33 



Strecker 1885 0-95334 106-32 



L. Lorentz 1885 095388 105-93 



Rowland 1887 0-95349 106-32 



Kohlrausch 1888 0-95331 106-32 



Glazebrook and Fitzpatrick 1888 0*95352 106-29 



Wuilleumier 1888 0-95355 106-27 



The specific resistance of mercury at o° C. is therefore 95352 

 C.G.S. units. 



The mean of the values in centimetres of mercury — 106-3 — 

 omitting Lorentz's, must be considered a very close approxima- 

 tion to the true ohm. We thus have 



B.A. unit 104-808 cm. 



Legal ohm ... ... ... 106 



Ohm 106-3 



The B.A. unit is thus 1-347 per cent, wrong. 



What is the specific resistance of pure copper ? is a curious 

 question to ask in 1888, but Mr. G. P. Prescott asks it in the 

 Electrical Engineer of New York. He points out that Ayrton 

 gives it as i'599, and Stewart and Gee I '616, legal microhms, at 

 the same temperature, o° C. He also shows that Matthiessen 

 and Jenkin did not agree ; they differed 2-3 per cent. Messrs. 

 Glazebrook and Fitzpatrick, who have done such good work 

 with mercury, might well turn their attention to copper. It is 

 well known that Matthiessen's standard for pure copper is 

 wrong. It was one English standard mile of pure annealed 

 copper wire 1/16 inch in diameter at I5°*5 C, having a resistance 

 equal to 13-59 B.A. units. It is a common thing to get copper 

 giving better results than this. 



The magnetic elements for 1887 as determined at Greenwich 

 were — 



Mean declination I7°47'W. 



Mean horizontal force 181*75 



Mean dip 67 26' 20" 



Why does the Astronomer-Royal retain British and metric 

 units when nearly all the world uses C.G.S. units ? 



THE MICRO-ORGANISMS OF AIR AND 

 WA TER. 



"C" VER since the great importance of micro-organisms in the 

 ■*■**' economy of Nature was pointed out by Pasteur now some 

 twenty- five years ago, the presence of these minute living forms 

 in the two great fluid media — air and water — with which we are 

 surrounded, has formed the subject of elaborate investigations. 

 As these investigations are thus co-extensive with the period 

 during which micro-organisms have been made the subject of 

 careful study, a review of them becomes particularly instructive 

 as illustrating the gradual development of the methods of 

 bacteriology from the earliest times up to the high degree of 

 perfection to which they have attained at the present day. 



It was Pasteur himself who first instituted a systematic inquiry 

 into the presence and distribution of micro-organisms in the 

 atmosphere in connection with his well-known researches dis- 

 proving the spontaneous generation of life. The experiments 

 which he undertook for this purpose are as remarkable for their 

 extreme simplicity as for the striking results which they yielded. 

 Thus the apparatus with which Pasteur set about exploring the 

 distribution of microbes in the air consisted simply of a number 

 of small flasks, each partially filled with a putrescible liquid such 

 as broth ; the necks of these flasks were drawn out and sealed 

 before the blow-pipe whilst the fluid contents were in active 



ebullition. The flasks thus prepared were both vacuous and 

 sterile, and could be preserved for an indefinite length of time 

 without the contained liquid undergoing change. A number of 

 these flasks were then momentarily opened in various places — in 

 Paris, in the open country, at various altitudes in the Jura 

 Mountains, and at an elevation of 6000 feet at the Montanvert, 

 near Chamonix. Each flask on being opened became instantly 

 filled with the air of the place in question, whilst, by sealing the 

 flask directly afterwards, the further access of air was prevented. 

 On preserving these flasks which had been thus opened, the 

 liquid of some was found to become turbid and lose its trans- 

 parency owing to the development of bacterial life within it, 

 whilst in others it remained perfectly clear and translucent. It 

 was further observed that the proportion of flasks becoming so 

 affected varied greatly according to the places where they had 

 been exposed. Thus, of twenty flasks exposed in the open 

 country near Arbois, eight developed living organisms ; of twenty 

 opened on the lower heights of the Jura Mountains, five became 

 affected ; whilst of the twenty opened at the Montanvert, close to 

 the Mer de Glace, only one broke down. The proportion of 

 flasks which became affected on being similarly exposed in Paris, 

 was considerably greater than in the case of the experiment 

 made at Arbois. 



The results of these simple experiments thus convey a most 

 vivid picture of the great density of microbial life in the air of 

 towns, and of its attenuation in the higher regions of the atmo- 

 sphere, although they can give no account of the actual numbers 

 present in the air under examination. 



Miquel and Freudenreich 1 made the first step in the quanti- 

 tative estimation of aerial microbes by aspirating air through 

 plugs of glass-wool, thus taking advantage of a fact long known 

 — that it is impossible for micro-organisms to pass through 

 sufficiently tightly constructed plugs of such materials. 



Without entering into a detailed account of this method, the 

 merits and demerits of which have been fully discussed by 

 German investigators, it is sufficient to state that a very large 

 number of experiments have been carried out by the authors 

 which can lay claim to a fair degree of accuracy. However, 

 since solid nourishing media for the cultivation of micro' 

 organisms were introduced by Koch, the importance of substi- 

 tuting the latter for the liquid media hitherto exclusively employed 

 has led experimenters to devise processes which shall render 

 their use possible in the examination of air. 



The advantages possessed by solid over fluid media are very 

 great, for whereas in fluid media, such as broth, the organisms 

 are in no way restricted in their movements, and their multi- 

 plication can take place indiscriminately throughout the entire 

 liquid, on the other hand, if they are introduced into gelatine- 

 peptone which has been first melted, they can be evenly dispersed 

 throughout the culture-material by gentle agitation, and by 

 subsequently allowing it to solidify they are not only isolated, 

 but rigidly confined to one spot. Thus each individual organism 

 becomes a centre round which extensive multiplication takes 

 place, and in a few days definite points of growth are visible to 

 the naked eye, which are appropriately described as ''colonies," 

 and which can be easily counted with the aid of a low magnifying 

 glass. Although each colony consists of many thousands or even 

 millions of individual microbes, yet as in the first instance they 

 owe their origin to a single organism or indivisible group of 

 organisms, it is correct to regard the number of colonies as 

 representing the number of micro-organisms. These colonies 

 have often very beautiful and characteristic appearances, 2 and 

 it is exceedingly remarkable how constant and distinct for one and 

 the same organism these appearances are. In many cases they 

 give rise to magnificent patches of colour — deep orange, chrome 

 yellow, brown, various shades of red, green, black, &c. Often 

 under a low magnifying power they are seen to spread over 

 the surface of the gelatine, producing tangled networks of threads, 

 sometimes they resemble the petals of a flower, sometimes the 

 roots of a tree or its branches ; in fact, one is constantly startled 

 by the novelty and beauty of their modes of growth. 



Koch 3 and, later, Hesse 4 have devised methods by which the 

 organisms in the air become deposited on a solid surface of 

 gelatine-peptone, and by there producing colonies render their 

 estimation possible. A large number of experiments have been 



1 "Annuaire de l'Observatoire de Montsouris," 1879-86. _ ^ 



8 *' Studies on some New Micro-organisms obtained from Air," Phil. 

 Trans., vol. clxxviii. p. 257. . 



3 Mittkeilungen ans de7ii kaiserlichen Gesnndheitsamte, 1881, Bd. 1. 

 * Ibid., 1883, Bd. ii. 



