“Why do chemists like nitrates so much?”/“They’re cheaper than day rates.”

A chemistry riddle is:
 
Q: Why do chemists like nitrates so much?
A: They’re cheaper than day rates.
   
The “nitrate”/“night rate” pun has been cited in print since at least 1922.
 
       
Wikipedia: Nitrate
Nitrate is a polyatomic ion with the molecular formula NO−3 and a molecular mass of 62.0049 u. Nitrates also describe the organic functional group RONO2. These nitrate esters are a specialized class of explosives.
       
7 April 1922, Illinois State Register (Springfield, IL), pg. 4, col. 2:
Heard in Congress.
Member of the Farmer Bloc: “Will this act make nitrates cheaper?”
Telephone Magnate: “Night rates are already cheaper than day rates.”
   
13 October 1924, Elmira (NY) Star-Gazette, “The Once Over” by H. I. Phillips, pg. 3, col. 3:
Johnson: I need a word pertaining to nitrates.
Harris: Night rates are about two cents a word cheaper than day rates.
 
4 February 1925, Austin (TX) Statesman, pg. 4, col. 6:
QUITE TRUE
Chemistry Professor: What can you tell me about nitrates?
Student: Well—er—they are a lot cheaper than day rates.—Illinois Wesleyan Argus.
   
Google Books
August 1926, Boys’ Life, “Think and Grin” edited by Frank J. Rigney, pg. 44, col. 2:
The Difference
CHEMISTRY PROFESSOR: What can you tell me about nitrates?
STUDENT: Well— er — they’re a lot cheaper than day rates.
   
Google Books
A Treasury of Science Jokes
By Morris Goran
Springfield, IL: Lincoln-Herndon Press
1986
Pg. 64:
Why do chemists use nitrates to make explosives? Because nitrates are cheaper than day rates.
 
Twitter
anotec‏
@olfactory
why do chemists like nitrates??,—-Because they are better than day Rates
11:02 PM - 10 Sep 2007
   
Twitter
k‏
@pfspleen
Q: Why do chemists like nitrates so much? A: They’re cheaper than day rates.
5:55 AM - 12 Mar 2009
   
Google Books
Teachers Joke and Story Book
Collected by Stanley Graham
Bloomington, IN: AuthorHouse
2016
Pg. ?:
Chemistry teacher: “What can you tell me about nitrates?” (sounds like night rates)
Student: “They’re a lot cheaper than day rates.”
 
Twitter
jolly dad jokes‏
@dadtellsjokes
Why do chemists like nitrates so much?
They’re cheaper than day rates.
7:15 PM - 13 Dec 2017