KNOW l.llx.l. 



June, 1912. 



microscope and seen in lif^iiiv J4J will l)c c-on- 

 siderod. Tlie solar rays are reflected from a 

 lieliostat tlniHif^li the slit A into the darkened 

 laboratory in which the apparatus is set up. 

 The li^ht falls on to B, a telescope objective, 

 l>y which it is concentrated on to the slit C, which 

 is capable of very fine adjustment. D is a screen 



tJ 



r:^ 



FiGl'RE 242. 



with an aperture of sufficient size to cut off any 

 stray ravs of light reflected from the edges of the 

 slit." .\nother objective. 1'2. directs the light into the 

 condenser F. from wliich it passes as an intense 

 beam into the solution contained in G. At right 

 angles to the path of light is arranged the microscope 

 H for minute examination of the track of light in G. 

 This latter cell, for the use of solutions, usually 

 takes the form shown in Figure 243, the rectangular 

 part K having (piartz faces and being fitted also with 

 a funnel and outlet, which permit of easy washing 

 and filling without disturbing the adjustment. 



The above design by Siedentopf of the ultra- 

 microscope is not the only one which has been 

 used. A simpler device, due to Cotton and Mouton. 

 is worthy of note. .\n oblique parallelopiped of 

 glass, surmounted b\- the solution and cover slip, 

 is placed upon the stand of the microscope. The 

 convergent illuminating beam follows the course 

 shown in Figure 244, the angle of incidence on the 

 upper surface being adjusted to lie between the 

 critical angles for water-glass and glass-air surfaces 

 and the beam, therefore, is totally reflected. Then, 

 the only light which enters the microscope H is that 

 diffracted hv the ultramicroscopic particles of tht' 

 solution. 



F'or the examination of larger particles, t'.jt;., cells. 

 bacteria, and so on, modified 

 forms of the ajiparatus may 

 be employed, in which, for 

 convenience, the illumina- 11 ^ 



ting rays and the rays 

 diffracted from the particles 

 are in the same straight line. 

 But as these would require Figuuk 243. 



complicated designs and 

 descriptions, their consideration is omitted. 



To understand at all completely the beauty of this 

 apparatus, resort must be made to a consideration 

 of several of its more interesting applications. Its 

 possible service to the colloidal chemist and to the 

 bacteriologist has already been hinted at, and we 

 cannot do better than turn our attention to at least 

 one of these ap[)lications. Before so doing, a few 



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-'-""I 



figures Midicating the degree of sensitiveness as com- 

 pared with other methods of analysis might be 

 deemed interesting. liy the aid of spectrum 

 analvsis, Bunsen and Kirchhoff consider that 

 (»• l4 X 10 '■ mg. of sodium can be detected : whilst, 

 in the case of hydrogen, Emich avers that as minute 

 a quantity as 0-7x10 '■' mg. gives appreciable 



indication of its 

 presence. Again, 

 it is asserted by 

 Fischer that by 

 the sense of smell 

 2-2 XlO"-* mg. of 

 mercaptan and by 

 Berthelot that 

 10~" mg. of iodo- 

 form are capable of 

 detection, though 

 here the examples are far and aw^y among the most 

 exceptional that could have been chosen. In the 

 chemical way, too, i X 10~' mg. of sodium hydroxide 

 are able to produce a definite change in the colour 

 of some suitable indicator. With the ultramicro- 

 scope, however, a particle of gold of mass 10 '" mg. 

 in a gold ruby glass ma\- be observed without 

 trouble, whilst when the most favourable conditions 

 are a\ailable, a particle of si^se ten times that of an 

 average chemical molecule will not escape observa- 

 tion. Obvioush', the potentialitiesof the instrument 

 are great and there is no saying what more astound- 

 ing results may be expected when the method is still 

 further improved. 

 .\ppLic.\Tio\ TO Tin-: Srrnv of Colloids. 

 No more fascinating field of enquiry is open to 

 science than the investigation of so-called colloidal 

 solutions. That such a substance as platinum, 

 for instance, generally regarded as insoluble 

 in water, should be capable of forming what 

 appears to be a solution, is certainly striking on first 

 acquaintance. Not only metals, however, but many 

 ■' insoluble " substances, like silver chloride, can be 

 made to develop this peculiar state by suitable 

 methods. Natural colloids are of universal distribu- 

 tion, the very constituents of living 

 cells appearing to exist in such a 

 form. The properties of a colloidal 

 solution afford a striking contrast 

 to those of ordinary solutions : for 

 instance, their slight ability to 

 tliffusc and consequent separation 

 from truly soluble salts by dialysis, 

 their insignificant osmotic press- v- 

 ure, their peculiar electrical Figcke 244. 

 properties which seem to point to 

 the presence of an electric charge upon the particles, 

 their capacitv for coagulation and absorption, and 

 so on. .\11 these circumstances add an interest 

 to the study of colloids which has served to attract 

 a great host of in\estigators, as will be gathered 

 from the enormous outiuit of work upon the subject. 

 Much controversy has been waged over the exact 

 nature of these colloidal solutions, for they bear 



