6io 



LECTURE XXXV. 



also from the fact that the aggregations of swarm-spores at the margins, just 

 mentioned, can be induced by placing the vessel with the one edge on a cold body 

 and the opposite one on a warm body: it matters not whether the light here 

 cooperates at all or falls in any given direction, or not, the superficial collection 

 of swarm-spores at the margin is always formed where the water sinks in thp 



vessel at the colder place. In like 

 manner the rounded groups mentioned 

 above, networks, concentric clouds, &c. 

 (Fig. 359 £), are produced quite in- 

 dependently of the light, when the tem- 

 perature of the vessel is the same on 

 all sides, by the bottom of the vessel 

 having a different temperature from its 

 surface, and particularly if the latter 

 is uncovered and evaporation takes 

 place; vertical currents are thus pro- 

 duced, ascending and descending in 

 the liquid, and the contained swarm- 

 spores collect at the relatively still 

 places. I was able to produce all 

 these phenomena in exactly the same 

 way by employing, instead of water 

 containing swarm-spores, a mixture of 

 water and alcohol in which coloured 

 olive-oil was distributed in the: form 

 of very small drops, simply taking 

 care that these fine drops of oil 

 were a little lighter or a little heavier 

 than the mixture of alcohol and 

 water. 



Quite apart from these currents pro- 

 duced by differences of temperature in 

 the liquid, and by which the swarm- 

 spores are passively carried and caused 

 to form the above-named cloud-like ag- 

 gregations in the vessel, the swarm- 

 spores themselves possess a peculiar 

 irritability for light, which consists in that 

 in their swimming movements, which 

 moreover occur also in the absence of 

 light, they pursue a definite course, and this so that they either move away from the 

 source of light, or in the contrary direction, and in the line of the rays of light. 

 If a large number of similar swarm-spores are contained in a drop of water under 

 the microscope, and if they are suddenly illuminated from one side, they all swim 

 towards the source of light or away from it. This is the phenomenon which 

 has been studied in detail by Stahl and Strasburger, and I may take yet another 



^IG. 359. — Emulsion-figures. Oil coloured witli allcauet was ttio- 

 roug^hly shaken up with 3 mixture of water and alcoliol of nearly 

 equal (somewhat greater) specific gravity, so that a fine emulsion was 

 produced ; and this was poured into a plate. j4 arrangement of 

 the oil-drops after a short time when one edge of the plate (the 

 upper in the figure) is cooler than the other. B when the plate is 

 equally warm on all sides, but the bottom of the liquid warmer than 

 the upper surface. 



