Find the chemistry joke in this crossword

ACROSS

1. Saw
6. Groom fastidiously
11. Texting icon
16. Copy
17. Stand for a sitting
18. Evita or Juan of Argentina
19. With 27-Down, 48-Across, 27-Down (again!), and 81-Across, droll observation from a chemist
21. Andean ancient
22. The “S” in STP
23. Companion to whiskey in “American Pie”
25. Father figures
26. Un número perfecto
27. Scented hair ointment
29. Software program, briefly
32. It means nothing in Nicaragua
34. Ear science
38. Canine cap
41. Stamp in The Collector
43. Subject, for 12-Down
44. Fill to the brim
46. Pilot
47. Hack
48. See 19-Across
51. Cow poke?
53. Soft balls
54. Captain Ahab or his ship
57. Voucher
58. Inquisition target
60. Candid Camera catchword
61. What a test tube might do during an exothermic reaction
63. “Physician, ___ thyself”
65. Partner of abet
66. Go by
68. Fiddler’s place?
71. Final Four game
74. Word before “top” or “dog”
75. Like benzene to an organic chemist, or like perfume to a nonchemist
79. Fidelio is Beethoven’s only one
81. Rash, or see 19-Across
83. Manages, as for oneself
84. Spooky
85. Burdened
86. “Lion’s Share” fabulist
87. Auditions (for)
88. Borax polymer in a fun chemistry experiment for kids

DOWN

1. X, Y, or Z
2. Adroit
3. ___ Stark (Game of Thrones protagonist)
4. Hired hoods
5. Radiate
6. Views, as through a microscope
7. ___ McNally (mapmaker)
8. Equi equivalent
9. DC line
10. Monopoly moola, e.g.
11. Prefix with gram or center
12. Genetics pioneer Gregor
13. Marine killer
14. The Grapes of Wrath surname
15. Taverns
20. Shower
24. Admit making a mistake
27. See 19-Across
28. Anonymous John or Jane
29. Make a scene?
30. Debate position
31. Appears suddenly
33. Tennessee Williams’s streetcar
35. The “thee” in “Get thee to a nunnery”
36. Prefix with centric or thermal
37. Chatter
39. Lumber
40. Gridiron org.
42. Minuscule, to a toddler
45. Final event?
47. Make final final preparations?
49. Final report, in many a college class
50. Sounds of hesitation, combining the symbols for elements 92, 1, and 16?
51. Babe in Babe
52. McCorvey in a landmark case
55. Lilly or Broad
56. One end of the spectrum
58. Shakespearean or theatrical Prince
59. Transfers to the next column to the left
62. Creep
64. Nonhelical element, in structural biology
67. Binge
69. Skips
70. Terminal
71. Company seat?
72. Pointless Olympic event?
73. “___ sana in corpore sano”
75. LSD, in slang, ironically enough
76. Part of a pickup line?
77. Agenda entry
78. Hand over
80. Nonspeaking part in Antony and Cleopatra
82. Old NOW cause