RECENT PROGRESS IN PHYSICS. 413 



of lalior in eacli kind of work. The fineness of the work at once ren- 

 dered necessary some economical reproduction of the plates, in order 

 that the maps might be sold at a low price, which would have been 

 impossible if a new engraving had to be made when each plate wore 

 out. The first attempts brought to light a most serious difficulty, 

 the copper deposit was liable to adhere, involving the partial destruc- 

 tion of the costly original. 



B}^ an ingenious process of reasoning Mr. Mathiot was led to the 

 employment of iodine to prevent the adhesion of the deposit to the 

 matrix. It was, however, found difficult to apply the iodine uniformly. 

 The plates were next, previously silvered, and then washed with an 

 alcoholic solution of iodine, which effectually produced the desired 

 result. It was soon found that when the iodized plate had been ex- 

 posed to the action of light the separation was most easy and certain, 

 and this is the plan now uniformly adopted with unfailing success. 

 For the largest plates a solution of one grain of iodine in twenty 

 thousand grains of strong alcohol is used. 



" To test the effect of the chemical method of preventing adhesion 

 on the sharpness of the engraved lines an engraving was seven times 

 successively transferred from plate to plate, when the closest inspec- 

 tion failed to show any inferiority of impressions from the last plate 

 as compared with those from the first." — (Coast Survey Report for 

 1851, p. 544.) 



The next matter of importance was to diminish the time and conse- 

 quently the expense of depositing large plates without impairing the 

 ductility of the metal. Previously the best results attained had been 

 the deposit of one pound of copper in twenty-foui' hours on a plate of 

 eight square feet, and this only under extraordinary precautions. 



The desired end was attained by a succession of improvements. In 

 the first place the platinized silver plates of Smee's battery, the one 

 which Mr. Mathiot seems to prefer, were liable to a coating of impu- 

 rities derived from tlie zinc, (zinc itself deposited upon them?) this was 

 found to be removed readily by immersion in a solution of chloride of 

 iron. In the next place, in order to bring the plates of the battery 

 nearer together, to reduce the resistance, it was found necessary to 

 have silver more flat and true than could be made by hammering. 

 Such plates were finally made by electrotype deposits, by means which 

 we cannot now stop to describe, but which insured certainty and uni- 

 formity in their manufacture. After these improvements Mr. Mathiot 

 was able so to proportion the size and number of the elements of the 

 battery as to attain the required rapidity of deposit without sacrifice 

 of the ductility of the metal. Finally, by a sim])le contrivance for 

 maintaining a constant temperature above 100° Fahr. in the decom- 

 posing cell, the rapidity of the deposit and the quality of the metal 

 were still more improved, with the further advantage of keeping the 

 saturation of the copper fluid more uniform throughout its mass. 



In order to maintain the exceedingly fine surface required by the 

 delicatfLWork on these plates, it was found necessary to make the first 

 immersion in a vertical ])osition, so that impurities in the solution 

 could not rest upon the plate upon which the deposit was forming. 

 To save the trouble and time required for drawing olf the fluid and 



