1832 - found online.
The word Laboratory is difficult to read, but it is in the lower right quadrant, between the long straight Washington street and the meandering Stony brook above it. This is two years before the laying out of the railroad tracks, but this map and the one below show a similar area.
The Map of Roxbury (John G. Hales) 1832 at the JP Historical Society web site give a much clearer view from the same year.
Sidney & Shields, 1853 (BPL)
The word "Laboratory" goes through the dotted line of the railroad tracks, just above the large capital "R" near the lower right corner.
Finally, this 1874 map from the JPHS shows Chemical avenue, today's Cornwall street.
I've been puzzling over a street name for a while. Cornwall street, which runs from Washington street to Amory street, was once called Chemical avenue. The 1874 map linked above shows Chemical avenue ending at Brookside avenue, where the Aetna Rubber Mills stood. By 1885, the Cable Rubber Co. stood on the opposite corner. Now the process of vulcanizing rubber is a chemical process, but I would think that Rubber street would have been a more appropriate way to memorialize the business than Chemical avenue. Soooo.... I kept looking. Or at least I finally got around to looking into Chemical avenue.
I believe that I've probably accounted for the "Chemical" name by starting with a notation on an old map, seen above. Both maps, published 20 years apart, show the word Laboratory. It turns out that there was a Norfolk Laboratory in Roxbury during those very years. I suspect that Chemical/Cornwall avenue was the original driveway to the factory from Washington street. In the early 1830s, Amory street, Brookside avenue and Green street had yet to be laid out, so this was an isolated location, with easy access to the Dedham Turnpike (Washington street). The articles below tell us that there was a fire at the factory in 1830, and that they were still in business in 1847.
A note on the first article: they were producing ether in 1830 - that's before the discovery of the anesthetic effects of ether. It was being used for medicinal purposes, but the anesthetic powers of ether were not recognized until the next decade, when Boston played a major role in it's introduction into surgical use. Also, a demijohn was a large glass bottle of up to several gallons, often covered with a weaved matting for cushioning.
And finally, I finally found something to deal with my chronic King's evil.
Essex Gazette March 16, 1830
A Fire broke out on Monday noon, in the Chemical Works of the Norfolk Laboratory, in Roxbury, from the bursting of two demijohns of ether. The fire spread so rapidly that the whole building, with its contents, was consumed. Loss estimated at 10 or 12,000 dollars, and no insurance.
Boston Daily Atlas August 4, 1847
CHLORIDE OF SODA. An Article which has proved highly beneficial in scrofula, king's evil, and most of the cutaneous affections, and as a disinfecting agent. Prepared at the Norfolk Laboratory, and for sale at No.5 Hichborn Block, Ann street, by O.W.F. MELLEN.
Additional source: History of ether.