TRANSACTIONS OF THE SECTIONS. 89 



remain in contact with the glass during its immersion in the acid. It is impossible 

 to etch deeply with hydrofluoric acid, in consequence of the acid acting laterally as 

 well as vertically as soon as it has removed the superficial layer. The process, how- 

 ever, yields results of great beauty, both with colourless glass, and with that flushed, 

 that is, covered by a thin plate of coloured glass. Copies of wood-cuts or engravings may 

 thus be produced in various colours ; and for windows and lamp shades, as well as for 

 decanters, toilet bottles, drinking vessels and the like, the method is readily and 

 cheaply applicable. 



Other applications will occur to every one who masters the principle of this process. 

 Wood-cuts are only preferable to other forms of engraving as giving broader lines 

 and yielding in printing ink an admirable protective varnish. 



The essential part of the process is the retention of the paper to the proof and 

 etching. 



On some of the Stages which led to the Invention of the Modern Air-pump. 

 By Professor George Wilson. 



The author began by stating that he had long ago proposed to himself the task of 

 illustrating the special service which the ' Instrument ' rendered to physical science, 

 as distinguished from the ' Idea ' or thought which guided the physicist in devising 

 and using the instrument, and the effect of individual peculiarity or idiosyncrasy in 

 affecting the interpretation of those phenomena and laws which the idea and the 

 instrument together brought to light. 



In the history of every science we recognize the prevalence at particular periods, of a 

 more or less comprehensive Thought, Idea, Hypothesis or Theory, which determines 

 the direction of inquiry during that epoch. Such, for example, in astronomy was the 

 doctrine that the earth goes round the sun ; such in chemistry the doctrine that com- 

 bustion results from the union of unlike chemical substances. 



These and similar ideas, however, even when most true, never reach mankind as 

 pure truth, the lumen siccum of the Divine Mind, but are always more or less modified, 

 coloured, or obscured by the idiosyncrasy of the human expositor who first gives them 

 utterance. The antecedents, accordingly, of such men as Copernicus, Galileo and 

 Lavoisier, the quality of their intellects and moral faculties, their education and 

 training, the fears and prejudices or hindrances under which they wrought, and much 

 else must be studied, before we can rightly estimate their influence over the progress 

 ofscience. 



Further, in the case of the physical sciences, and especially the experimental ones, 

 a third and notable element, affecting their progress, appears in the character of the 

 instruments by which the phenomena they are concerned with, are observed, tested, 

 analysed, and registered. Thus Galileo's telescope furnished every man with ocular 

 demonstration of the truth of the Copernican views; and Lavoisier's balance for ever 

 extinguished the ignis fatuus of phlogiston. 



A comprehensive history of any physical science must include all three elements: 

 viz. 1. the dominant idea of a given epoch, which, in so far as it was true, was the 

 recognition of an actual law of nature or thought of God's ; 2. the human expositors 

 of this idea, who in converting it into a formal doctrine, always more or less modified 

 it, and often in consequence retarded or misdirected the progress of knowledge for 

 centuries; 3. the instrument realizing or applying the idea or doctrine; in some 

 cases justifying a hypothesis, in others pointing to a theory, — in all bringing physical 

 phenomena much more within reach of the observer. 



The author thought that the historians of physics had too little regarded the im- 

 portance of the last element of influence. The telescope, the microscope, the baro- 

 meter, the thermometer, the air-pump, the electrical machine, the galvanic batterv, 

 the electro-magnet, the photographic camera, and many other instruments, has each 

 added a new kingdom to the map of science, and a new chapter to its history. 



In communications addressed to different learned bodies, the author has referred 

 to several of these instruments ; the object of his present paper is to indicate some 

 important points in the early history of the air-pump. 



In relation to instruments intended to produce a vacuum, we may conveniently 

 regard vacua as of four kinds : — 



