232 APPLIED SCIENCE 



potash, water, citrate of iron, and ammonia. The solution 

 is applied with a camel's-hair brush and is then allowed 

 to dry. After drying, the paper assumes a greenish yellow 

 color. 



The blue-print itself is made in the following manner. 

 The tracing of the drawing is placed over a piece of blue-print 

 paper. The two are then put into a frame constructed simi- 

 larly to an ordinary picture frame. The frame is then exposed 

 to the direct sunlight. The rays of the sun pass through 

 every portion of the tracing paper except the black lines 

 of the drawing, and act upon the chemical solution of the 

 blue-print paper in such a way as to turn to a yellow color 

 the entire paper, with the exception of that portion beneath 

 the black lines. The blue-print paper is then dipped into 

 water, which changes it, with the exception of the lines, to 

 a blue color. The lines become white and are, of course, 

 an exact reproduction of the tracing. 



Questions 



1. Describe the composition and manufacture of a lead pencil. 



2. What constitutes a good grade of drawing paper? 



3. Explain the composition of an eraser. What qualities must 

 an eraser possess? 



4. How is rubber obtained and refined? 



5. What is a working drawing? 



6. What is the difference between a working drawing and a 

 perspective drawing? 



7. What is tracing cloth? 



8. What is tracing paper? 



9. Describe the composition of India ink. Why is it used for 

 drafting purposes? 



10. What is a blue-print? How is it made? 



