EXPERIMENTS WITH AIR FURNACES. 233 



They should not be made in part of cast-iron and 

 in part of wrought-iron ; as it is proved that where- 

 ever the two come together, there oxidation goes 

 on with great energy. In the summer months, 

 when furnaces are not in use, the ashes deposited 

 about the joints become moist, and wherever thin 

 iron pipes are connected with cast-iron shoulders, 

 there the work of oxidation goes on, and the whole 

 is soon destroyed. This energetic action is due to 

 galvanic currents, resulting from the different crys- 

 talline structure of the metals. 



After patient practical trial of two of the best and 

 most popular forms of furnaces, it was apparent that 

 inventors and manufacturers had not yet supplied 

 apparatus which was not open to serious, or even 

 fatal, objections ; certainly no one that we had ex- 

 amined could be permitted to remain in operation 

 in our dwelling for even the briefest space of time. 



The objections or defects may be stated to be : 

 first, the employment, of cast-iron in the interior 

 construction ; second, the presence of joints in the 

 air chamber, which cannot be made perfectly tight 

 by lutes or cements ; third, arranging the air 

 chambers or flues so that ashes and dust can pass 

 into the air current ; fourth, the imperfect arrange- 

 ment of smoke flues and dampers, by which great 

 loss is incurred from incomplete combustion ; fifth, 

 needless complication, and consequently needless 

 expense in construction. 



