660 Transactions of the Society. 



Generally speaking, in all chemistry, acids hinder decomposi- 

 tion, tending to hold water and other matter in a state of tension. 

 Thus we use acids for pickling, in the same way as we use strong 

 neutral saline solutions ; that is to say, mixtures of water with 

 substances having a strong attraction for it. In the experiments 

 recorded acids come to check movements by neutralizing alkali, 

 just as Professor Jevons has shown that they arrest Brownian 

 movement when chemically indifierent matters are suspended in 

 pure water. I may say that before the publication of Dr. Jevons' 

 observations I had made many experiments to test the influence of 

 acids, and that my conclusions entirely agree with his. In stating 

 this, I have no intention of derogating from the originahty of 

 Professor Jevons, but simply of adding my testimony to his on a 

 matter of some importance. Free alkalies, on the other hand, are 

 everywhere provocative of decompositions and unrest. Beyond 

 what we have noticed, as in peptones, we see alkahes used by the 

 chemist to induce breakings up, oxidations, and the formation of 

 new matters, usually of smaller out of larger molecules. They 

 tend, in fact, to determine the formation of new matters with which 

 they may combine, and so help to favour a tendency which seems 

 to belong to all matter, that of coming to rest. 



From these various considerations I venture to draw the inference 

 that minute intestine vibrations of the colloid suspending fluid have a 

 power of intensifying molecular movements in suspended particles. 



This is, I submit, greatly favoured by the concomitant varia- 

 tions set forth, showing that the movement of particles is more or 

 less active according to the presence in the surrounding fluid of 

 conditions favouring or hindering chemical changes in the colloid. 



I may cite an experiment in which the method of difierence 

 gives results in the same direction. One of the best mixtures for 

 demonstrating molecular movement is that of iudian-ink with dis- 

 tilled water. The solid ink is rubbed gently with water until a 

 mixture of suitable thickness is obtained. This consists of jjarticles 

 of solid black matter suspended in water now dissolving whatever 

 viscous matter had been used to bind the particles into a cake. 

 What this viscous matter may be I know not exactly, but from all 

 appearance it is some kind of gum — in other words, a colloid. 



Now, if a large quantity of indian-ink be rubbed down with 

 water, and the mixture be left in a tall vessel to allow of subsi- 

 dence of particles, the viscous matter may be in process of time to a 

 large extent washed away, leaving a mixture of particles with nearly 

 pure water. This mixture shows an infinitely less active and per- 

 sistent movement, though with particles of the same size and number. 



The influence of solutions of soap upon Brownian movements, 

 as set forth by Professor Jevons, appears to me to support my con- 

 tention in the way of agreement. He shows that the introduction 



