[Illustration: Fig. 71.] We may now continue our account of the fermentation which we were studying when we made this last digression. On June 17th that fermentation produced three times as much gas as it did on June 11th, when the residue of hydrogen, after absorption by potash, was 72·6 per cent.; whilst on the 17th it was only 49·2 per cent. Let us again discuss the microscopic aspect of the turbid liquid at this stage. Appended is the sketch we made (Fig. 72) and our notes on it:—“A most beautiful object: vibrios all in motion, advancing or undulating. They have grown considerably in bulk and length since the 11th; many of them are joined together into long sinuous chains, very mobile at the articulations, visibly less active and more wavering in proportion to the number that go to form the chain, or the length of the individuals.” This description is applicable to the majority of the vibrios which occur in cylindrical rods and are homogeneous in aspect. There are others, of rare occurrence in chains, which have a clear corpuscle, that is to say, a portion more refractive than the other parts of the segments, at one of their extremities. Sometimes the foremost segment has the corpuscle at one end, sometimes at the other. The long segments of the commoner kind attain a length of from 10 to 30 and even 45 thousandths of a millimetre. Their diameter is from 1-½ to 2, very rarely 3, thousandths of a millimetre.[145] [Illustration: Fig. 72.] [Illustration: Fig. 73.] On June 28th, fermentation was quite finished; there was no longer any trace of gas, nor any lactate in solution. All the infusoria were lying motionless at the bottom of the flask. The liquid clarified by degrees, and in the course of a few days became quite bright. Here we may inquire, were these motionless infusoria, which from complete exhaustion of the lactate, the source of the carbonaceous part of their food, were now lying inert at the bottom of the fermenting vessel—were they dead beyond power of revival?[146] The following experiment leads us to believe that they were not perfectly lifeless, and that they behave in the same manner as the yeast of beer, which, after it has decomposed all the sugar in a fermentable liquid, is ready to revive and multiply in a fresh saccharine medium. On April 22nd, 1875, we left in the oven, at a temperature of 25° C. (77° F.), a fermentation of lactate of lime that had been completed. The delivery tube of the flask, A, (Fig. 73) in which it had taken place had never been withdrawn from under the mercury. We kept the liquid under observation daily, and saw it gradually become brighter; this went on for fifteen days. We then filled a similar flask, B, with the solution of lactate, which we boiled, not only to kill the germs of vibrios which the liquid might contain, but also to expel the air that it held in solution. When the flask, B, had cooled, we connected the two flasks, avoiding the introduction of