INVERTEBRATA, CRYPTOGAMIA, MICROSCOPY, ETC. 1045 



arranged in tho three dimensions of space, and that the diameter of 

 the molecule will be approximately as the cube root of the number of 

 atoms it contains, so that a molecule of alum will be equal to 



(VIOO = 4-64) iruV^DW = ?7TTJ7FTT mm., 

 and a molecule containing a thousand atoms will have a diameter of 



A good Microscope will enable a skilled observer to identify an 

 object so small as the a- .T.nr vaxn. Beale, in his works on the Micro- 

 scope, pictures some fungi as minute as that ; and Nobert's test bands, 

 and the markings upon the Amphipleiira pellucida, which are about tlio 

 same degree of fineness, are easily resolved by good lenses. If thus 

 the efficiency of the Microscope could be increased fifty times 

 ( '""""" = 50), it would be sufficient to enable one to see a molecule 



400 ■" _ _ 



of albumen ; or if its power could be increased one himdred and 

 seven times, it would enable one to see a molecule of alum. 



Now, Helmholtz has jiointed out the probability that interference 

 will limit the visibility of small objects ; but suppose that there 

 should be no difficulty from that source, there are two other conditions 

 which will absolutely prevent us from ever seeing the molecule. 



1st. Their motions. A free gaseous molecule of hydrogen at the 

 temperature of 0° C, and a pressure of 760 mm. mercury, has a free 

 path about ttiooo '^™- ^^ length, its velocity in this free path being 

 1860 m. per second, or more than a mile, while ils direction of move- 

 ment is changed millions of times per second. Inasmuch as only a 

 glimpse of an object moving no faster than one millimetre per second 

 can be had, for the movements are magnified as well as the object itself, it 

 will be at once seen that a free gaseous molecule can never be seen, not 

 even glimpsed. But suppose such a molecule could be caught and 

 held in the field so it should have no free path. It still has a vibra- 

 tory motion, which constitutes its temperature. The vibratory move- 

 ment is measured by the number of undulations it sets up in the ether 

 per second, and will average five thousand millions of millions — a 

 motion which would make the space occupied by the molecule visibly 

 transparent, that is, it could not bo seen. This is true for liquids and 

 solids. Mr. D. N. Hodges finds the path of a molecule of water at its 

 surface to be • 0000024 mm., and though it is much less in a solid, it 

 must still be much too great for observation. 



2nd. They are transparent. Tho rays of the sun stream through 

 the atmosphere, and tlio latter is not perceptibly heated by them, as it 

 would bo if absori)tion took place in it. Tho air is heated by conduc- 

 tion contact with the earth, which has absorbed and transformed tlio 

 energy of tho rays. -When selectivo aUsorption takes place, tho 

 number of rays absorbed is small, when compared with tho whole 

 number presented, so that practically tlie separate molecules would bo 

 too transparent to bo seen, though their maguitudo and motions were 

 not absolute hindrances. 



Wale's "Working Microscope." — Tho now feature of this instru- 

 ment by Mr. G. Walo (Fig. 115) consists in tho method of suspending 



